This doesn't appear to be quite the same issue as #720 or #738. Using v10.0.0.

I have a modal component that only renders itself when active, and as part of this modal component I use FocusTrap. When in Strict Mode, opening the modal causes onDeactivate and/or onPostDeactivate to get called immediately.

The focus trap works perfectly fine either without Strict Mode or without being conditionally rendered.

Here's a barebones CodeSandbox demo of the problem.