I've been having this problem for months. I can't remember exactly when it started and I'm not sure what causes it. I keep hoping I'll see a cause but nothing seems reproducible.
My host is Windows 10 Home and my guest is Windows 10 Pro. While working in the guest in Full Screen suddenly the mouse capture will be lost. At this point I minimise the guest, click the Show button in Virtual Box Manager and click once with the mouse and I'm back again.
It's irritating and I'd love to know how to fix it. I've googled but not found anything that helps.
I'm not even sure if it's triggered by something happening in the guest or the host.
Using VirtualBox v6.1.12 but it also happened in v6.1.10 and possibly earlier versions.
Iain