I'm curious to read a bug report; I never noticed file copying be any issue on FreeBSD but maybe there's certain methods to run into it
Iirc I saw that 14.2, and kmods-latest was introduced 14.3; I'd be using STABLE or CURRENT and having kmod-latest repo to avoid that, but if drm-kmod is flaky I'm kind-of wondering the expectation of FreeBSD on desktop/GUI (seems STABLE/CUR would be a requirement unless drm-kmod is immediately built/available with new REL)
That's not new or FreeBSD-specific; on Linux it's a guess whether it's pkill thunar vs
Thunar 
(and why I do large transfers with Terminal cp -R)