This isn't a problem we've seen in house.
Keep in mind that GarageBand on the macOS and GarageBand on iOS are fundamentally different programs, so stability on one platform doesn't mean stability on the other. It might be worth seeing if the crash happens w/o any other AUv3s or audio apps open on your iPhone – that way you can pinpoint if it's GarageBand, or another program causing the crash.
And as always, being on the latest iOS and GarageBand might help.
