So this is probably (definitely) not supposed to happen.
If one play with the mixed team Chaos god selection (chaos and nurgle) you get access to an apothicary and players with decay.
Just for kick, I tried a few games vs 11 rotters to bash them around to see how decay and apo work together. My tabletop friends were all baffled to the question, so I had to check!
Result: If one use an apo on a rotter, it allow you to reroll the 1st injury... and to IGNORE the 2nd one. The game simply give you a choice of the original injury and a new one, and whatever is chosen is the ONLY result that stand. No 2nd injury!
Its a consistent bug that happened every time and can be easily tested in a couple of minutes.
Obviously its a suuuuper important bug that need to be fixed ASAP as we all regularly use apos on rotters
Not sure if it was reported before, but if not, this bug shall forever be known as the Cyric's bug