My visitor cap was already adjusted down to 20 in all cases, even before I encountered the bug, because I value my FPS.
My current fort seems to be plagued by the visitor bug as well, but not as severely as the previous ones.
I am getting about 1-2 visitors arriving every dwarf day instead of 5-6.
Next to that, they are leaving as well, at about the same rate they are coming in.
I've only seen it go above the 20 cap twice in the first season of my fort, and only 22 max. Hopeful.
It might be related to me designating a 3x3 tavern instead of the 7x7 and 6x6 I used before, then again it might not be. Gonna test to see if a bigger tavern matters once I've got enough blocks to finish that outdoor tavern (I want to designate it from a table and not set it as a meeting hall, and keep my dwarves in a 3x3 area to hopefully get married, so it needs to be indoors).
Maybe related, maybe not, I've only got contact with 2 civs instead of 3. Elves went extinct during worldgen.
My previous fort that did not have the bug did have elves, but did lack access to goblins, so it also only had contact with 2 civs. Might be completely random coincidence though, since the bug is present after all in this fort, while it was completely absent in the other one.
A good indicator of your new fort having the bug is visitors arriving in the first year, even before you have designated a tavern/inn.
If they do arrive early, designating an outdoor unwalled inn from an outdoor meeting area can quickly confirm if it is indeed bugged, and save you the frustration of only finding it out after setting up your basic economy.
Still sucks to lose the time investment of worldgen, searching for an embark location, and preparing carefully this many times in a row. This is my 8th fort in a row suffering from it, although I might just not need to abandon this one. The other ones were just crazy, with +5 visitors every dwarf day from the moment you open an inn. That just means FPS death before the end of year 2.
BUT I SHALL PERSEVERE
EDIT: Don't think it's going to hold. Visitors up to 32 out of 20 in autumn year 1