I was able to reproduce some bad keyword autogeneration, like getting keywords from URLs inside media tags.

I can also note that you won't be able to set keywords when adding entries regardless of the automatic extraction option IF you have the "Enable SEO fields" option set to "Only when editing".

However, I can't see how keywords would generate automatically when the feature is disabled, and I cannot reproduce it or reproduce keywords not deleting from catalogue entries when manually removed. My best guess is your browser is doing some autofill on the keyword fields.