JabRef 5.1--2020-05-04--b5599c9
Linux 4.9.0-12-amd64 amd64
Java 14.0.1

Changes in the entry preview format are accounted for only after restarting.

Note 1: may seem similar to issue #5464, but the present issue is about the content of the preview.
Note 2: issue is present with "Show preview as tab in entry editor" checked or unchecked.

Steps to reproduce the behavior:

1. Create a file with the following entry and save it

@Article{MyKey,
  author    = {Author1, Surname1 and Author2 Surname2},
  journal   = {The Journal title},
  title     = {The title of the article},
  year      = {2020},
  number    = {12},
  pages     = {1-100},
  volume    = {15},
 }

2. Go to Options -> Preferences -> Entry preview
3. Select "Preview" in the column "Selected"
4. Click on the Edit tab
5. Change
   \begin{journal} <em>\format[HTMLChars]{\journal}, </em>\end{journal}
   into
   \begin{journal} <em>\format[HTMLChars]{\journal},HereIsMyChange </em>\end{journal}
6. Go to the preview Tab.
   ==> The preview is updated accordingly.
7. Click on save
8. Edit the entry
   The preview shows:
   
   HereIsMyChange is not displayed ==> The preview is NOT updated.
9. Close and reopen JabRef
10. Edit the entry
    The preview shows:
    
    HereIsMyChange is displayed ==> The preview is updated NOW.