Hello Yra !

It would be nice to have more details form your request:

I can’t add a custom field the first time. The system header is erased, you need to enter it again. 

If you mean the trouble from the given screenshots - it's UNA rule of work to fill captions if the site has more than 1 installed language.

If you still manage to add, various database errors may occur when editing the field on the site. 

Need some examples, but if the users already filled the new field then better not to change the type there.

There was also a case when I deleted the system header of the html block, but failed to return it back, I had to do all this in the database. 

Good to see the details too.