Host is Devuan Beowulf (Debian Buster) with all the latest updates.
Guest is Devuan Beowulf, also with all the latest updates.
Host and guest are running vbox 6.1.26 software.
Host video is VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Cedar [Radeon HD 5000/6000/7350/8350 Series] (from lspci on host)
Was running guest in full screen mode
Using XFCE desktop menu -> Logout -> Hibernate
Screen went dark (as expected) but control does not return to the host video
Tried Host-F to escape from the full screen mode (by then, probably too late)
Tried getting a host console with Ctrl-Alt-F1 but nothing happened
(also tried Host-F1 in slim hopes that maybe vbox would cooperate, even though by then it was clearly hibernated...)
Tried to connect to host from a test box hardwired to host in question, but I did not leave myself any means to log in... (but I guess my security is OK)
Restarted the host in complete frustration.
For comparison, performing all of these steps when NOT in full screen mode works fine. I was able to hibernate and restart the guest.
I admit maybe there were other steps I could have tried, but I couldn't think of them.