What I don't understand is why Moscow doesn't just admit to what they're doing. Everybody knows anyway, and no one is going to do anything about it. But it's insulting they still try to maintain this charade of "soldiers on vacation". Those "vacationers" "loaned" entire battalions of the Armed Forces of the Russian Federation, for heaven's sake. The documentation is in this very thread.
It begs the question why they don't advance further, and I think that
@Talibanhunter hit the nail right on the head. Either outcome – victory or defeat – would create new problems which Mr. Putin might not want to have on his hands, but it's advantageous for Russia to maintain the current status quo. It prevents Ukraine from seeking further integration with the West, and it sends a strong message to other states of the collapsed former Soviet Union not to poke the bear.