Hyperpigmentation in oral mucosa.

So this is happening since may after i took frst covaxin jab. I have been to multiple dentists and ents, they saying its normal they checked it physically, initially it happened in buccal mucosa now its happening in labial mucosa as well. I have been prescribed topical steroids before but when used it reduced it a bit then later it aggravated after stopping. A dermatologist said its olp but m confused as all dentists are saying its normal. Should i ask my dentist for biopsy? Also i had some dry mouth issues last mnth too, idk why but buccal mucosa felt dry. They said its not melanoma but melanosis.. bt now its also happening in my labial mucosa and when i lick it it feels sour. M worried as its not reducing and if it has a potential to turn into cancer. I am non smoker, no tobacco usage. Please help.. i dnt have any itching or burning, just that area feels a little rough like sandpaper and sour to taste