Assuming banner is truly random there is insanely low odds that this happens coincidentally.
Ever since EA release the banner has only rotated out 1 single unit at a time 2 stay consistently at least until the next banner. Thats why its common to see the same banners its a 25% chance that the banners unit gets swapped out with the same unit meaning the banners stay the same 25% of the time. If this is an intentional game design I apologize for wasting your time making you read this ( would be stupid if it was intentional tho).
IF BANNER WAS TRULY RANDOM
6 mythics
1,2,3,4,5,6 if previous banner were mythics 1,2, and 3
it would be 3 separate rolls on banner first roll being 50/50 of either being 1,2, or 3 of 6 (1/2 odds)
The same unit cant be rolled twice so lets say first roll was 1
There is now 2,3,4,5,6 which is 2/5 odds to roll same unit from old banner because the rolls are dependent to one another
Say it rolled 2 from old banner again now we only have 3,4,5,6
Rolling 3 is now 1/4 odds which is the same as the current odds because the banner doesn't worry about the first 2 events occurring
Ultimately making the chance of the same banner being 1/20 odds ((.5)x(.4)x(.25)=.05) General multiplication rule (Multiplies probabilities to find new probability given previous event occurred)
yeah that's all. I think the general consensus is the same that 25% seems a lot more like what is happening than the 5% if the banner was truly random.

