Every mysterious die off has a reason, except that it has not been identified. I can recall 4 die off incidents in my 40 year of fish keeping First, when I lived in Florida, my water company switched from chlorine treatment to chloramine. I didn't put enough dechlor and the fish died off in front of me before filling up to the top. A second time I placed an ornamental silver shining rock I bought from a rock shop on a trip. The fish slowly died off in days apparently from heavy metals leaking off the rock. The third time I glued an ornament onto a rock with silicone. I was careless not to soak the the cured silicone in water overnight before placing in the tank, thinking that a small amount of silicone in a big tank won't matter. It didn't wipe out all the fish, but selectively my entire colony of Tang compressiseps. The forth time I decorated my tank with plastic ornaments I bought from mail order, the type made for fish tank. All my small Tangs started to die off in days, but my bigger Malawian or CA/SA were unharmed. I suspect the plasterizer in the ornaments did it, because after I removed them, the die off stopped. In all 4 incidents, the killer was toxin in the water.
I can bear witness that a mis managed canister can release toxin. My broker friend has a small 20 gal tank in his office filtered by an oversized canister. He told me he only cleaned it once to twice a year. But the oversized canister is so powerful that it sucks in fish food so he had to turn it off temporarily during feeding. One time he forgot to turn it back on over night. The fish were fine the next day without any water circulation. But a couple hours after turning teh canister back on, all the fish were dead. He opened up the canister and discovered strong sulfide smell and a lot of trapped food.
Yes, BB are resilient and don't die off easily and this is why companies are selling BB in closed cans. But in the absent of oxygen, the BB are no longer beneficial and stop converting ammonia to nitrate. Instead, the same bacteria can switch to anaerobic mode and digest organic matter by stripping oxygen off nitrate and sulfate turning them into toxic nitrite and sulfide. If the trapped waste in the canister is mostly fish poop, it's no harm because fish poop has no food value. But if the trapped waste contains substantial left over food, it's a time bomb.