tinfoil hat on
An overall 33% drop rate and variable drop rate throttling at several specific tresholds that don't statistically affect the overall drop rate aren't necessarily mutually exclusive. I track my farms too and they've always been in the expected average ~33% drop rate, and I have definitely had stretches of what I intellectually dismiss as bad RNG when a toon was at 9X/100, but what definitely seems at first glance like the game deliberately throttling the drop rate for a couple of days at the tail end of a farm to incentivize me to spend crystals on shards.
tinfoil hat off
There is no evidence for this, but to my knowledge there is no concrete evidence that would outright disprove it, either.