Could you elaborate or point me to some explanation ? I'd like to understand the rule behind it.
I suppose it's not as simple as "if a mech can overheat it can't have AC", because there are mechs like that - eg. https://www.masterunitlist.info/Unit/Details/1771/king-crab-kgc-0000
I'm no expert in the rules, but, I believe it has to do with how overheating reducing the damage of an item in the conversion rules.
So if you managed to overheat far enough, the AC might be down to less than the X-Damage it needs to score at least 1 point of Damage at the AS Scale.
This is why some designs have a 0* Special or something like that. The damage is too low to even count.
The KingCrab is packing Twin AC20s so it would take a LOT of reduction to not count them.
Where as the Banshee-3S is only a Single AC10 on a mech stuffed fill of PPC/ML/SRM/SL Spam.
You'll notice it also doesn't have an SRM stat either.
Essentially the 3S can do 58? Damage at Medium Range but only has a Damage stat of 4 w/ 2 OV.
The missing damage is enough to reduce the AC/SRM to not being their own separate stats.
I don't know the exact math but if you look into the conversion rules it should be there.
You'll also notice the KC you mentioned only has an AC Stat of 2, not 4 for the total of 40 damage it can do in ACs.
This is due to the Overheat reducing "average" damage.
Bracket Fire mechs tend to lose specials compared to Alpha Striking mechs with full HS capacity.