Merciless RNG sucks.
Before I retired, it happened in September/ October/ November 2018.
Also reports of back to back and ones in multiple slots at the same time.
Officially rare wanted elemental monster chests were never nerfed, but if they were nerfed ( not saying they were ) it was probably when 4* heroes were buffed to only need 3* ascension items. The Devs love merciless RNG but data mine to make sure certain targets are not exceeded ( pity/ mercy timers for staying in an alliance would have discouraged mercing without pissing off the mercs ).