If you are using strict mode, it is a known bug. On GitHub, I explained that the bug is due to the new cross-site cookie isolation introduced in 86. This is enabled if you use strict mode or set it in custom. This is a bug in Firefox Webextension API itself and it is out of our control.

Hello. We have manual clean buttons in the popup. Click the yellow 'clean' button to trigger cleaning of sites that aren't in open tabs or configured lists. If you click on the down arrow next to clean, you get additional cleaning options, the first would be 'clean, include open tabs', which would simply clean all sites not configured. Note that if you have any sites configured to greylist, they will still be kept until restart, as part of the configured lists.