OK, did another test, this time with Safari Technology Preview and Safari: Both of them had numerous tabs that were dormant (not brought to foreground) since yesterday evening. For both apps I could now (~18h later) save and restore all tabs with the macro.
The 18h include the system sleep time over night. Safari was hidden all the time, STP not.
So, there are chances that this really has been fixed (I’m on OS 10.14.6), which would be astonishing though. Or it needs still more dormant time to trigger the issue, or other factors (e.g. low on memory). Don’t know.