From @philsward in https://github.com/backdrop/backdrop-issues/issues/4829#issuecomment-781...
Also, both Themes and Modules need a "remove/delete" option to completely delete them from the file system. Once a theme or module is installed through the GUI, there is no way to get rid of it from the GUI.
GitHub Issue #:
5320
Recent comments
Thanks, the /tmp folder was the problem. I moved it elsewhere and then the updated worked. Thanks again!
Update 1.35.1 appears to install but hasn't
Probably not the reason, but worth to do a quick check: Sometimes the update cache interferes, so if something's odd with updates - some module (or core) claims to have updated, but then...
Update 1.35.1 appears to install but hasn't
Nope, not touched any permissions!
Update 1.35.1 appears to install but hasn't