Thanks to you too for your help!
Now it works on the original lot too, (same game save, same family, same lot) but in between the testing I had bulldozed the lot and changed the build (My temporary fix was to have a common hall with only one exterior door connecting both the business and house door). I rebuilt it more or less to how it was in order to test, but perhaps the bulldozing did something. The bug doesn't appear in my original save anymore, or new one neither.
Thanks for the link, glad to know I'm not alone! But scared it will happen again loll
But in the link it's front door vs back door, in my case both door were side by side in front. Only difference it seems.