Gamarleton, my pleasure. Thank you for letting me know of the issue!

Gamarleton, I've just finished updating the bathmats. I adjusted the box colliders so they're 1cm smaller. That should minimise collisions so there should be less movement. The trade-off is that anything placed directly over a join might try to place directly on the ground/surface below the mat, however I think the gap is narrow enough to avoid that for most items. New rugs should be out soon-ish.

Gamarleton, there might be something I can do with the mats to make them less triggering for collision so I'll take a look at them next week. I also originally intended to make some larger versions... and since I can't remember why I didn't, I'll add those to my to-do list for when I get some free time.

That would be due to the game's physics engine. Each item has a 'box collider' that controls how other items interact with it. Two items' box colliders touching tends to trigger a physical collision, which causes the items to shift around (it happens all the time with the game's original objects too). The collider for the mats covers the whole mat (as that makes the whole mat selectable for moving or deleting), so putting two side by side could trigger movement in one or both of them. Usually, it only happens if you move or sell one of the touching items after placing it. I'm sorry you've been having this issue. The easiest way to avoid it is to use the mats singly, however, if you make sure each mat is exactly where you want it before putting it down (so you don't have to move it again), it should have less movement issues. That should also stop the rug from going under the floor.