It's pretty clear that NATO does not want to get militarily involved in the war between Russia and Ukraine. However a NATO country like Poland or Romania could still decide to send in troops to defend Ukraine on their own accord.
Let's say something like that were to happen and Russia starts bombing Poland or Romania in retaliation, would Article 5 still be invoked in such a case? The troops would only be sent in to help bolster the defense of Ukrainian cities. They would not engage Russian troops unless they were being attacked by Russian troops. Nor would these troops attack any targets in Russia or Belarus.
So if Russia decides to attack Poland/Romania for merely helping to defend the territory of Ukraine, would Article 5 still be invoked?