I finally found out what caused the issue in my game. As far as I remember, it came up with the november 2020 (PC: 1.68.154.1020) Patch. After buying Outdoor Retreat this year (2022), the issue was solved for me. All lots from gallery causing this issue in my game used the foundation that came with Outdoor Retreat. Now I can download them without missing walls, missing foundations and floating furniture. But, I guess, buying a pack can't be the solution, so EA should have a look at this. I'll try to report them.
Sorry but there is nothing EA can do. It is up to you to check what packs are used in a build. If you only have the basegame you should look for things that do not require other packs. Since you now have Outdoor retreat you can include that.
I just wrote it for people (like @chocolatemuscle and @sicitur), who have the same issue, so at least they know where it comes from.
I just don't understand why the OR Foundation isn't replaced by a basegame foundation, because that's what I expect to see. Just like a chair, window etc. would be replaced with basegame stuff.
