Why sometimes Cyrillic is saved to the mysql database and sometimes \u58fgrf \4urj using ajax?

Question

PHP

It seems there are constants:
utf-8
JSON.parse, JSON.stringify, json_encode, json_decode
But anyway, sometimes it saves Cyrillic. If, for example, I change the data on the front and re-save the data with Cyrillic, it again becomes \u58fgrf \4urj. How to bring to a single standard and what is it?