Custom field "entrytype" resets upon modification

Thank you @ThiloteE , here is a way to replicate the second issue using fresh library.

1. Create a new library
2. Go to Options -> Customize entry types
3. Select Article and click on the drop down list. Please note that there is no Entrytype field available in the drop down list.

4. Type in Entrytype and click on + button

5. Create 2 new articles
6. Navigate to Optional fields.

7. Delete the value in Entrytype at Optional fields
8. Select second entry and reselect first entry
9. The Article value is back in Entrytype at Optional fields. However, it is being deleted in bibtex source.

My first guess to this issue is that there is an internal EntryType variable that does not allow user to modify it.
However, I could confirm that this second issue does not relate to #8527.
Would really appreciate if it could be moved as separate issue ticket.

Originally posted by @LIM0000 in https://github.com/JabRef/jabref/issues/8527#issuecomment-1161631732

Start in Options → Customize entry types and Optional fields using the fresh-library reproduction steps. Confirm how deleting Entrytype behaves when switching between entries, then inspect the BibTeX source result. Done means a user-edited Entrytype stays changed in Optional fields and is preserved in the BibTeX source.