If it isn’t getting changed it wouldn’t be conflicting.
If a program is using the PST it will likely make changes to it, they are not just flat files but containers of items. Syncthing is not designed to synchronise these when in use by two machines at once, that would require purpose built software, and this could lead to data loss.
One being master should prevent anything major happening so…
In the advanced settings for each folder you can change the number of conflicts to keep. The default -1 is unlimited, I assume 0 would be none.