I know, it defies logic. I am running Manjaro KDE, and have tried this with Chromium, Brave and Firefox, and with all three browsers I get the same result. The only reason I can think this setting is getting deselected is because the signatures tab has no signatures listed, hence my original post on why I was not presented with the "download signatures" button, and how to go about populating this signatures tab with some entries, as otherwise ClamAV can't do its job (as I understand it).
This means they are already found and you maybe had this plugin installed before.Check the logs if sigs are updated