Apparently a severe memory leak: https://old.reddit.com/r/laravel/comments/16yr8rb/why_were_c...

The PR in question: https://github.com/laravel/framework/pull/48078

Thanks for those - turns out, I'm just lucky and have been using in a manner this never reared its head in my (only one) application where they're used. Heavily. But with the way things are done - the tagged caches were only flushed during a time that there were never going to be new keys added.