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Got a build going on, and left the braced top and back plates over the holidays.
Top and back over the were weighted (1" MDF board and no 7 plane) and between 1" foam on a radius dish, 25', soundboard on the bottom, foam, back, foam, board and plane.
Top's fine, but after carving the back braces (parabolic, not scalloping the ends) I noticed the tops of the braces were convex, not flat as they had been.
Checked the radius against a 15' radius stick, then a 25' - not even 25' radius! (Both plates were checked after bracing against the radius sticks)
Workshop is temp/humidity controlled/insulated and I checked it every day more or less (3 calibrated hygrometers, all agree)
There was some variation over the period, from around 38 to 50 deg RH, but not for long, a few hours at most.
Not, I thought, enough to cause this. It was always brought back to 43 % quite quickly.
And I've done this with other builds before while working away for several weeks, with no ill effects.
Top 2 braces (Adi) are not too bad, but the lowers ones (Sitka) are out of order.
Trying dampening the outside, bending and clamping a reverse curve, then SS and hot air gun and weighted in a 15' radius dish, again with a reverse curve.
They have moved a bit, I'll see in the morning, but strongly suspect the two lower braces will need replaced with freshly radiused ones.
Why have the Sitka braces been so badly effected and the Adi not so much?
The Adi was heat treated as it was fairly fresh, but the Sitka was cut into over sized rough blanks 2/3 years ago and has been it the shop ever since.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Your back is the problem, cross grain movement is greater than with the grain, back is moving, bracing is not. Get everything dry then glue, you might need a "hot box"

Sorry, not understanding.
I understand the cross grain/moving/bracing not moving bit.
The back is dry, it's always been 50% or less RH in the shop.
Glued up at 43%, little bit up and down.
+/- 7% - would that be enough to cause a "permanent" set in the back radius?
If by a "hot box" you mean a humidity/temp controlled environment, I've got one - as I said, the whole workshop.
Trouble was we had big temperature swings over the holidays and the heating takes some time to react/adjust.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Well something moved.....and since it has flattened it must be the back shrinking a bit (cross grain movement). I had a very nice sycamore back braced up and during a cold snap built a fire in my basement shop wood stove and it did a very radical reverse curl I was amazed.

Sure did!!

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

You probably won't get them to go back to their original curve....This is coming from the same experience. Your best bet is to shave the braces off and re-brace what you need to.

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BTW, it was dryness that caused this, not damp, just in case anyone is mislead.

I now recall I set the dehumidifier too low on evening to dry out the shop a bit - forgot and left it on overnight.
RH was right down in the morning - lesson learned!
Shop first, beer after.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Yep

You were quite correct.
Braces all duly ripped off (well, planed/chiseled off) new set all (all Adi) ready to go today.
No point in half doing it

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Exactly the same thing happened to me last winter holiday, same results, same fix... Glad you got it sorted!