Dear crinrict,
I know this has been an ongoing issue for many players judging from various forums and comment sections. It took me a while to find this bug report here, because it does not have the word "restaurant" in its title.
I own at least one restaurant in each world (apart from the destination worlds) and was able to narrow down the problem to any townie that was generated in Selvadorada! Everybody else will come to work as usual, except for the Selvadoradians. So from 60 employees altogether, 13 of them were from Selvadorada (I know this because I renamed all their surnames to be "Selvador") and none of these 13 ever show up for their shift, 100% of the time. This includes chefs, waiters and hosts. I find these numbers to be too precise to be considered coincidental. Everyone else is fine, including Sulani.
I usually just lurk, but since it's been a long time already and I have not seen anyone else reporting on this, I thought I finally needed to give you a heads up. (I consider my save file private and not shareable, thus sorry about that.)
I hope this helps a little and thank you so much for your tireless work!
