I have a bug that seems a bit trivial but may come up occasionally. This occurred about 10 days ago; sorry I don’t recall the time.
I had a 3* hero (Valen) who was fully leveled but had only 5/8 special ability. I got four Valens to level him up and used them at the same time. The game said that he had a 100% chance of increasing his special ability, because it sums four 25% chances. However, in this instance his special ability stayed the same!
I suspect what happens is that the game uses the level up characters sequentially, so that you get four 25% chances in a row. The chance that you will increase your special is then 1 - (1 – 0.25) ^4 = 68%. Within this 68%, there is the chance that you will increase the special two or more levels but that does not need to be mentioned.
This is a rare event but some people may be angry if they see their 100% chance unfulfilled. It really only is important if you use one or more of the same hero as is being leveled.