Until a credible intelligence agency weighs in it's a bit in the air.
On one hand this being a false flag is believable as it would greatly benefit Russia, because they're losing and nothing bar full mobilisation is going to turn the tide for them at this point and Putin needs something like that to justify it.
On the other hand this being a genuine Ukrainian attack is also believable as it would benefit Ukraine, if it was them and they deny it most people will believe it's a false flag and the louder Russia claim it isn't the more people people will be sure it is, if Ukraine believe Putin is certain to call for full mobilisation eventually then it's in their favour to trigger it sooner rather than later as the wests interest in this war is vast but not infinite (if anyone doubts that, just tell me what's happening in the Syrian civil war or in Libya right now without Googling).