Change of launch behaviour with recent core update ??

I have a launcher that launches an editor.

The editor itself is capable of merging multiple instances so that separate launches with different source files get displayed as tabs within a single instance.

This used to work without problem - but recently I have noticed that launching a second (or 3rd) instance causes the second to run as a separate process and not get merged.

The Prortable.ini file has the following set :-
DirectoryMoveOK=yes
SingleAppInstance=false
SinglePortableAppInstance=false

Nothing has changes recently in the app or the editor - so I am at a bit of a loss to understand what is going on.

Has anything changed recently in the main system that might explain this ??

TIA
Dave