I did say with Crimea its a special case.
As for eastern Ukraine no one has proof of Russian involvement. As long they don’t formally cross the border, they can do what they want.
The Donbass is not pro-Russian. The way they see it in Moscow, its more advantageous keeping it a thorn in the West’s side.
They can claim to be the knight in shining armor that they would be robbed of in an invasion. While Russia won’t be stopped by sanctions, it figures it has no need to intervene. Ukraine is being kept weak and that’s good enough for Russia’s interests.
Citizens of the RF (civilian "protesters" and troops) are in Donbas. Ukrainian SBU was getting tons of them. What additional proof of Russian involvement can there be? For Putin to formally admit it? He won't. If you recall, he didn't admit to Russian troops taking over Crimea.