I'm on my third playthrough. In both prior playthroughs I did not run into this particular ambush. Though sadly it seems third time is a charm and this time myself and my arabian got shredded in about ten seconds or less. No missions active or bounties simply riding out of Saint Denis from the stables. All seems well suddenly on the left hand side of the road some damn Lemoyne raider starts mouthing off about where I do and don't belong. I find myself stuck in some sort of slow movement lock very much like you experience in camp. The enemies appear around me ,one enemy shouts "get that SOB!" another shouts "open up that maxim on him"
I had zero chance to run or get myself to any defensible position before I was chewed to pieces by the enemies and the heavy machine gun they had used to kill me and my horse in the ambush. There was no "BULL IN A CHINA SHOP" bs going on I was casually riding out of town ,nothing new done it a million times this same route before and never been ambushed in THAT spot with so many enemies plus a maxim machine gun. I will agree back up save files help but in this situation short of making a save every dang time I ride into Saint Denis there was no way to expect that in the town itself.
Very much like wildpoop has stated you cannot evade, avoid or survive this encounter . It is a zero chance scenario and extra saves are pointless unless you plan to do so EVERY time you go in Saint Denis because there is NO way to know when or what triggers this event but when it hits you're dead no if ands or buts about it and so is your horse. No let me be clear I don't mind getting my backside kicked if I have at least a chance to fight back. Yes the risk of losing your horse adds an edge to the game but this isn't a "risk scenario" it's a "Oh well too bad you and your horse are dead" scenario.
TL:DR It was a Forced NO CHANCE scenario and mechanics that activated in the ambush made it feel very much like a huge snarky FU from R*. If it's a bug well sh*t happens and moving along but if this is a working as intended mechanic scripted intentionally that's pretty messed up.