I'm interested in this issue and will take a look at the build @ju1ka. @crinrict poodles? No dogs allowed in restaurants, don't you know.😃
Edit: So I had a chance to test, and there is definitely a mystery here. I placed your build on 3 separate lots, MOO on and MOO off @ju1ka. Before getting into the heart of the matter, I did notice a couple things I thought I'd give you a heads-up on: 1) the front door to the restaurant was removed when I placed the lot. I think it's due to the two trellises you placed on either side of it. With MOO off, the game would not let me place a door in that position unless I deleted them. 2) the host/hostess was attired in swim wear. This gave me quite a few chuckles to be honest, but I'm not sure if it's what you intended. Maybe it's because I'm missing an outfit from a pack that you have but I don't?
Moving on to the seating, every time I tested it, when my sim showed up to your restaurant, sims would already be present and seated outside. My sim was usually seated upstairs (2 out of 3 times) but was seated outside once. A few other sims were seated upstairs, but the majority were indeed seated outside and none were ever seated on the first floor. I tried removing the second floor and stairs to see if that would change the behavior, but it didn't - all sims were then seated outside. I tried deleting all of the décor objects in the outdoor area, just in case something out there might be somehow advertising it as the priority area, but that didn't change the behavior either.
I decided to look at some other restaurants that have a roughly similar design. I placed Updated 1950s Diner by lilsimsie and Le Grand Lama by Fezet. Neither have a second floor, but I don't think that matters given that your restaurant still had the behavior when its second floor was removed. Both restaurants have indoor and outdoor dining and neither are on a foundation, like yours. However, the behavior was the exact opposite of your restaurant. I never had a sim get seated outdoors with either of them.
I think this is a case where the devs may need to take a look at your particular build by downloading it from the gallery themselves and/or by taking a look at your save. I am out of ideas as to what else I can test. The behavior you're experiencing does not seem like it's the norm though, that's for sure.