So, Dine Out has the problem too? I didn't touch Dine Out for a long time so don't know the recent situation.
In my save, the "Staff missing" issue can be fixed by inviting them to the lot manually before opening the restaurant. If you can't see the "invite to the current lot" from the employee icon in the relationship tab, try using MCCC to invite.
There are many problems that I have to fix on-site. And many of the issues I forgot that it's fixed because of MCCC settings or minor mods. So, when I suggest the solution I know, I have to mention that it may need mods.
BTW, from what I remember before on hiatus, your restaurant's progress is so amazing. I hope you're doing well during this situation.