Not sure if it's the same thing, since I do not own the Royalty pack, but my married sims are also reverting to "partner" over and over. This happens with many different households, and I am not playing with any mods.
Interestingly, sims that were dating but then decided to just be friends are ALSO reverting to partners upon loading. This does not happen with sims that have broken up. Basically, all sims that were ever in a romantic relationship and don't have a negative romance bar become "partners", even if the romance bar is now at zero (the romance bar itself never changes in any cases).
I've been fixing them all manually so far, but it's been going on for a while and getting pretty annoying since I play rotationally and currently have over a dozen households I go through sometimes and every time I load in, or even sometimes when I don't, and am just in the middle of live mode, this happens.
For instance, most recently, I have a sim who completed the serial romantic aspiration, then became "just friends" with all but one of her partners, who she then married. Upon loading, she was back to being partners with all of them. So, I had her once again end all the the other relationships and then re-marry her spouse...only to find that as soon as the marriage completed, I was bombarded with notifications that she had simultaneously "become partners" with all her exes again. They weren't even on the same lot at the time lol.
So...yeah. Please fix this.