I'm having the same issue as of last night as well. Any kind of text formatting (bold, underline, font color) will disappear most of the time but not all of the time by navigating away from and back to an entry. This is mostly on entries where I created multiple entries per day (or month), or when the text is copied and pasted, the formatting often does not stay. Also, when I change text from a color back to automatic, the text color does not actually revert until I navigate away from and back to the entry.
If I start a new entry and type new text and format it, the formatting stays.
There is also a new behavior where, if I put the cursor somewhere in the middle of the page, the page will scroll to the bottom. It scrolls back to the middle while I type, but then scrolls to the end again when I stop typing. This is new behavior, it definitely did not do this previously.
I will share a screen capture with you privately if you provide your email. I'd rather not upload video of my private diary on a public forum, I'm sure you understand.
I did swap to a new phone a week ago, but it was fine up until last night.
Edit: Swapping to GBoard (Google Keyboard) appears to have resolved this issue, I will keep testing. So it may be related to the Samsung default keyboard.