I’ve only filled maybe 5 diamond tier chests myself… never even gotten a 3* unfarmable from them, let alone 4*.
I know. Tiny sample size. It’s hard for me to stay in diamond though. I work my butt off to get up there to open a chest and the loot is underwhelming. Nothing better than what I could already get in platinum.
If you’re getting a 4* mat from every diamond chest, you must be very lucky.
EDIT: as for the confirmation bias… I never got excited about pulling tall boots from any chest ever. Early on, my typical elemental chests would yield me something like a farsight telescope, a compass, an EHT, an ETT, and a common token. That was about average. Sometimes even better, sometimes slightly worse.
Lately I’ve been getting some elemental chests that only gave me tall boots and red books (not tome of tactics, the other farmable one) and 3 common summon tokens.
Just bad RNG I suppose.