After upgrading to CiviCRM 5.43 we've noticed that some fields are blank when performing an export after searching for activities. This is something we do regularly and was working fine before the upgrade.
The answer is to ensure that you select 'With' when you perform the search. Doing this ensures that related contact data is available when you export the resulting data.
Prior to CiviCRM 5.43, 'With' was selected by default. From CiviCRM 5.43 onwards, 'With' is not selected so you need to select in while doing the search.
The change was made in this PR: #21595
I have also found a bug in regard to this where the Display Mapping only shows the Assignee whatever option you choose. The export is correct however. Tested on DMaster Demo 5.50 alpha
I have created an issue Issues Log