When there's a batch of recently imported photos that I start adding keywords to, the "None" option doesn't appear in the Library Filter bar's Metadata browser Keyword column until you restart the app. This bug has gone on for some time, and it's a pretty big annoyance as someone who uses Keywords a lot.
[See this screen recording for steps to reproduce the bug in LR 10.1.1 / Mac OS 10.15.7:
Updates to Lightroom Classic and the Lightroom Ecosystem products for Desktop, Mobile and Web were released today and contain a fix for this issue. Please refresh your Creative Cloud application and install your update when it becomes available. Thank you for your patience.