:What you were expecting to happen, and what actually happened:
The guild, Voidwalkers, got our guild chest to tier 6, maxed out, last night [This was Saturday, March 13, 2021] and I checked the loot drop rate and spent over 70k seals and got 6 mythics. The screen shot showed: Troops 90% - Rare 64.8%, Ultra-rare 19.8%, Epic 4.74%, Legendary .64%, Mythic .02%; Traitstones 10% - Major 7.5%, Runic 2.1%, Arcane .35%, Celestial .05%. Today [This was Sunday, March 14, 2021], I was about to post to the guild the loot drop rates, and instead of checking my screenshot, I checked the rate in-game and and found the following better drop rates: Troops 90% - Ultra-rare 75.6%, Epic 12.4%, Legendary 2.25%, Mythic .11%; Traitstones 10% - Runic 9%, Arcane .8%, Celestial .2%. I haven’t found anywhere that stated loot drop rates change over time, so I am hoping this mix-up can be fixed so that we all can receive the proper loot drop rates.
:How often does this happen? When did it begin happening?:
You should not be seeing any rare troops at chest level 6—the OP looks like the drop rates for chest level 5. When you spent your 70k seals, did you get any rare troops?
Edit: 70,000 seals is 3500 chest pulls. At a 0.11% chance of mythic per pull, you should average 3.85 mythics given that number of seals spent. Your 6 mythics are nicely above expectation, and far more than you should have seen if the game were using the level 5 numbers during your spending spree. I believe this is a case where the numbers being shown haven’t been updated on the client end, though they are correct on the server side.
When you have the game open and you reach 40k, the game won’t immediately update the rates. You either have to do a battle or restart the game. Not sure if that’s what happened to you but I’ve seen it before in the above situation.