I had the same issue, after downloading the game I found in the source code that the choice for poly/open will fire an event as "partner" which is exclusive (can only have one at a time), instead of "poly/open partner" which is not exclusive as you would expect.
Basically in the current version it will only work the first time when you have no partner - I played around a bit, updating the code and running a modified version where the question fires the appropriate "open/poly" option I had it working as expected.
I'm unsure at the moment on how to report this to the developer, maybe it will be fixed in a next version.
They might even already be on it, it was easy enough to spot after spending a little time to figure out the basics of the code structure
Edit: I reported it and it was instantly marked as checked, maybe next version/build :)
Well, now I have a mistress, but the problem is that she doesn't fit the polycule apparently, so when I make my policule dates I only have one partner. I feel like that should change, or that if there is enough romance with your dom that should be taken into account somehow and the relationship should be a little different since it's not just a D/S relationship.