As I posted on JB, somewhat edited:
The Russian response is unfortunate, but it is quite understandable.
Russia, Syria's main ally, said it was also halting communications with the US aimed at preventing air incidents.
The situation is a many vs many with a variety of people backing various factions and thus creating chances for error/getting in each other's way. (see the Turk/Russian issue of a shoot down last year)
A variety of forces on the ground are getting air support: except for air support by SAF for Assad's forces, all air support seems to be sourced from various outsiders. Given the mess that this whole thing is, I'd say that something like this was bound to happen. (The Turk F-16 on the Frogger was a clue).
Assad doesn't trust "the West" with much of anything, so an overture like "hey, let's all gang up on the ISIS crew and sort the rest of this out later" is a likely no-sale transaction. Therefore, the many vs many continues.
You are the pilot of a fast jet, and you see some aircraft attacking your ally from the air, or setting up for another target run? What do you do?
I wonder if there was a better way to deal with this than "weapons free" given that (I think) this happened in Syrian Airspace (more or less). Probably not.
I suspect this engagement fit a clear RoE.
President Trump authorized the Tomahawk attack on a Syrian air base (some months back,
vis a vis the chem weapons attack, reported to be from that base) and I suspect that (given the SU-22 being the delivery platform for that event (alleged)) someone's logic chain at the CJTF (or in DC, since Micro Management is alive and well) was "we are preventing a gas attack" or something like that. Might just have been "stop the attack on our allied ground forces."
It's A Big Freaking Mess that Just Got Messier.
