I have experienced the same thing with my alt character who has received both the achievements for finishing "100 runs above Stage 10" and "50 runs above Stage 30" and have not yet received a single mythical accessory from AD (no ring, no earring, no necklace).
After hitting "100 runs above stage 10" achievement, I have manually started counting the number of runs and I am currently sitting at 152 runs without any accessory.
On my main, I started doing AD on the second day of release and the mythical ring dropped at Stage 24 while climbing with a pug group. Climbed all the way till Stage 41 without seeing an Earring or Necklace. But I eventually had both of them drop on Stage 31 on the second week which took roughly 150 runs in total to get all three accessories.
This made me question the drop rate as well, if the drop rate was indeed nerfed recently. Not seeing a single accessory drop even after 152 runs (with both achievements completed) feels ridiculous.