I'm not sure if this is a bug but more of a balancing issue. Mercenary troops aren't recruitable, so it follows that mercenary parties don't assemble parties in the same manner as other lord parties (i.e. recruiting from settlements). I assume they get free troops in some manner, like companions used to.
This of course is a balance issue. With the recent changes to mercenary contract deciding, one nation tends to hire every single mercenary faction on the continent, which is an upwards of 40+ parties. I captured over 40 party leaders in 3 battles yesterday, over half of which were mercenary party leaders. They were all gone from my prison within 5 days, and very soon were flooding me again with parties of 11-25 soldiers. The soldier counts are low, but the sheer number of parties has some consequences:
1. Raids go off the charts. In the war I was in, the Battanians (who only had a 2000 man force vs our 9000 man force, and they only had 1 town) had 65 successful raids while we had 4. The mercenary parties were constantly raiding every single village on cooldown, and they couldn't be stopped because they are incredibly numerous with very small parties that are capable of wiping out a village militia and also escaping from any party that tries to engage them. Then, as soon as one party gets chased off, another resumes raiding the village in its place seconds later.
2. Map flood. There are almost always 10-20 individual enemy parties following me around during sieges when at war with a weaker power due to this mercenary flooding issue. This isn't necessarily a balance problem, but it becomes quite annoying very quickly. And, like I said before, they seem to almost instantly escape imprisonment and come back with parties of 10-25 soldiers. Having 10 of those following you around at any point (with high speed because of small party sizes) is in fact a balance issue.
I haven't experienced the problem you have in regards to mercenary parties only targeting the player, though. I've definitely seen them going after a number of other targets, namely raids. It could be that due to the sheer amount of parties, you're perceiving them as only targeting you. There are, after all, 40+ of them running around all over the map.
It may however be a bug or unintended consequence (or oversight, even) of the last patch with the changes to mercenary contract decisions. It seems common sense that the larger nation with over 30 fiefs could pay more money than the nation with 3 fiefs (only one of which is a town). Yet, the nation with almost no income is paying every single mercenary band in Calradia to fight for them against a massively more powerful enemy? You would think they would get discouraged and discontinue the contract after a large amount of subsequent losses. Or that the more powerful country, which is surely richer, could pay them more. Of course, that exacerbates the snowball issue (which this honestly directly helps to counteract).
Perhaps a limit should be imposed on the number of mercenary factions a kingdom can hire, or provide more incentive for competing kingdoms to outbid for mercenary factions when a single nation is hiring many of them. This would prevent one single kingdom from amassing every single merc faction on the map.