I've play-tested this issue for 20 hours. I investigated the bug in 4 save files -- 2 new save files, the save file that I started when I installed the Businesses & Hobbies EP, and a save file from a time before owning B&H. The 2 new save files were never exposed to Mods as I've removed the Mods folder from The Sims 4 folder. The other save files had previously been exposed to Mods, but not at the time of research.
All save files exhibited the same bug. If a Sim never visited a Small Business Venue, the Sim was able to attend a Meetup. The Meetup would be normal, with spawned objects and visitors. If a Sim visited a Small Business Venue, they would no longer be able to attend any Meetup.
If a Sim is a Small Business Venue owner but has never visited a SBV, they can still attend Meetups. But if the owner of a SBV visits a SBV, they can no longer attend Meetups.
This includes Sims that live in the same neighborhood where Meetups occur.
This includes Sims that have never been an active household BUT have been spotted at a SBV. This occurred greatly in my B&H save file. I was play-testing Sims that I never played prior but had seen at a SBV. When playing them, they could not attend Meetups.
Here is an additional strange occurrence that I observed but I am not sure if it is 100% related. I was sending several Sims to a SBV. Once they arrived, I would exit to the world map and choose another household to send to the same SBV. If the Pancakes household was already at the SBV and I would go to the world map, back to their house and select a household member to play, both the Bob and Eliza icon would be faded. The game would load to the SBV, BUT they would not be inside, outside, or anywhere. Their face icons would be faded as if they were in a rabbithole location. I continued to research this and sent several households to the same SBV and each household would appear to be in a rabbithole and no longer in the SBV. They would be unplayable. Finally, I would send one more family to the SBV and all of the "rabbithole" Sims would rush out of the business as if they were exhausted, hungry, etc.
Edit By crinrict: Added pack tag to title.