I have the same problem, and it's definitely a new problem. One of my sims relatives runs a business on her home lot, and until recently I was able to visit as a guest as well as a customer. Now the business is always open even when I arrive as a guest. My sims will mostly do the business activities rather than regular guest activities, too.
It was also previously possible to hold dinner parties on small business residentials just like on normal residentials, because the business would be closed (I did that a few times on Winterfest). This most recent Winterfest, all the party guests except for the children and the toddler had to buy the entrance tickets at the ticket kiosk to be able to enter the house and partake in the party, and then most of the NPC guests didn't go into the private areas but kept to the business areas and the garden. I had to move the festive tree into the yard so everyone would actually come light it together. And because the business was open, random NPCs constantly came to talk to the party guests or tried to eat the food. I have held the same party on the same lot with the same sims before and this was never an issue.
