Steps to reproduce the issue

1. Make sure Joomla! has set timezone other than UTC - for this example i used Europe/Warsaw (+2h it's important )
2. Create custom field with type calendar with time
3. Make sure the field is displayed on article details - i added it after title
4. Create article and set custom field for desired date and time - for this example let's assume it's 20:00
5. Save & close the article
6. Check value of custom field added in Database - should be set to 18:00
7. Check date displayed on article on details view - it should display time we set while creating article
8. Edit this article again and change only category
9. Save & close

Expected result

Database value of custom field should still show time 18:00 and for sure it should still display as select time while creating article.

Actual result

Database value of custom field increase by 2h everytime category is changed which also changes displayed data of this field

System information (as much as possible)

Tested on Joomla! 3.9.19.
Server and database time zones are Europe/Warsaw

Additional comments

Sorry for bad English. it's not my native language.
Looks to me as if custom field always retrieves correct data from database but when category change is taking place something adds to the value 2h (probably time zones difference, I did not check that). You can clearly see that between steps 8 and 9.