Ideas
I just had this issue in my game after I started a brand new save; I had not experienced this in my old save.
For the first few hours that the business was open, everything was fine. Then a customer walked through a planter and into the 'Residential' area.
Once that first customer walked through a solid object and into the 'Residential' side, other customers stopped being limited to the 'Public' area and made themselves at home. They started entering into different parts of the lot that are marked 'Residential' and 'Employee' areas. I checked multiple times and the areas were still marked correctly in Build Mode.
When I closed the business some customers went into the residential area instead of leaving the lot.
I do have cc/mods, but I have never expereinced this issue before. All my mods are updated/working, and I have not installed any new mods right before this occured. It only started after I created the brand new save (which I only did because Roberto Crinkletop was completely missing from my old save - I think he died and no replacement was generated).