I happened to come across the same, or a similar, problem.
I have 3 files (differently processed versions of same picture): P818.jpg, P818-1.jpg, P818-2.jpg .
I want them to be sorted by name, in this order. XnViewMP under Windows does that

However, under Linux (Mint Mate), sort order in file browser (Caja, Nemo) is as intended, but in XnViewMP, order is always P818-1.jpg, P818-2.jpg, P818.jpg , no matter which "Sort by Name" option I choose.
Any way to change that?
Best regards,
Bezier