[2020] [Gameplay][IRON HORSE RAID] So now we get mortars in the tunnel on EVERY phase after the first?

OK, we used to get mortars coming into the tunnel after we'd damaged the boss (no idea how they managed to get in, but let's ignore that), and it was part of the challenge. We've heard all the old wives tales ('don't go into the tunnel until she runs off', 'throw a decoy out front, that draws mortars'), and followed the instructions, but since yesterday's update we're getting ridiculous mortar action, literally every time we line up, and multiple mortars. I raid pretty much every day, my team has people who raid every day, and multiple times, and all have remarked on the same behaviour since yesterday from their runs.
a) what has been done?
b) why, what was being fixed?
Please don't ask me for a video - it would seem pretty easy to repro (simply do boss 4).