What you were expecting to happen, and what actually happened:
Used 3 keys expecting ONLY troops, matching the drop rate description (75% 2 epic, etc.) And got traitstones in addition to troops. Used keys with the ever so slim increase in odds of acquiring a mythic (specifically the new one being advertized).
How often does this happen? When did it begin happening?
Once and i cannot replicate it having no additional VIP keys. Today is the first I’ve noticed it, having these keys only on rare occasions.
Steps to make it happen again
You would have to replace my keys for me to attempt it again.
It’s working as intended. In addition to the troops each VIP chest also contains 2000 gold and a 30% chance for a traitstone. Unfortunately, the help articles have been outdated for several years now, even after community reminders.
Hmm… from the screenshot, he only obtained 1 copy of Tyri (and Urielle The Guardian) when the tagline on the VIP chest screen clearly says “2 Epic Troops”. Which might be expected of 2 VIP keys, but he said he used 3 – which is verified by the Gold received. But also this isn’t the full list of rewards (per the scrollbar) so there’s probably 2 or 3 items not shown by this screenshot which would explain the rest.
The extra gold and chance for traitstones were tacked on later on, to improve the value of VIP chests. Unfortunately, most such changes seem to involve a generous amount of duct tape, so this never made it into the drop rate table (and probably can’t even be displayed as part of the drop rate table). At least the Game Guide has finally been updated to reflect the current handling.