>It occurred to me to test whether the problem occurs if form B isn't modal, and the answer is no, it doesn't. However, I need form B to be modal, because I need form A to wait for the user to dismiss form B to do some calculations. But still good to confirm that form B being modal is part of whatever's going wrong here.
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>Tamar

Tamar, you may consider to turn form A invisible or disabled while form B is presented to the user.