Control-V does not work in classic visual editor

This is strange. When I’m typing in the main visual editor of Classic Editor and I do a Ctrl-V paste, nothing gets pasted. Something happens, because the field moves a little, but nothing is actually pasted in. I can paste using the browser menus, but not with Ctrl-V. I can copy with Ctrl-C, and I can cut with Ctrl-X, but I cannot paste with Ctrl-V.

OTOH, I can paste with Ctrl-V into other fields on the same screen, including the title field.

This is happening in Firefox on Windows. I have Firefox running in Safe Mode (no plug-ins), and I have tried a local WP sandbox with no other plugins besides Classic Editor, and it still happens. If I disable Classic Editor, then I can paste with Ctrl-V in the Block Editor.

Any ideas?