Sorry if there’s a thread/faq that mentions this already but I couldn’t find it, how do I stop threads from un-bookmarking themselves? This didn’t use to happen, now after a while all threads auto remove the bookmark, halp 
Do you mean bookmarks in your browser?
Maybe listing your devise, os and other such details will help us figure out what is going on.
I use the app on my phone(android), and I bookmark threads here on the app at the bottom of the thread but the bookmark stops showing randomly, I have to delete and bookmark again for it to show
That seems to be an issue with the browser app on your phone. I do not know myself. Perhaps another knows more than I do.