It would not surprise me if Isis did this, nor would it surprise me if Russia allowed it to happen or were even behind it.
The false flag theory makes a lot of sense, but also does allowing the event to happen. Both give the Kremlin what they wanted, a rallying cry to support mobilization and continuing the war.
The “small” amount of egg on their face for not stopping the attack will soon be forgotten by the public with the cry of save the motherland drowning any criticism out.
Apparently, the Russians had received warning that this was going to happen.
Putin may use this incident to mobilize more troops and send them to Ukraine.