this is a special case and there are much worse in this game, but nevertheless this is a bug.
My example is for Sentinel Corrupt Rescuer.
If she has an existing Attack Up buff (2 rounds) and then gets a Block Benefitial Effects debuff, next round, she still has the Attack Up (1 round). So far so good.
If she then casts another Attack Up for 2 new rounds, this should be blocked, but the remaining 1 round should not be impacted.
However, when she does cast that, the Attack Up is removed altogether. Internally, I guess the old buff is removed and before it can be replaced by a new buff of the same type but with longer time to live, the blocker is in effect.
I guess this is valid for all events where the lifetime of an existing buff is to be prolonged. Instead of casting the effect as new buff, only the lifetime of the old buff should be extended (or in this case the extension be blocked by the debuff).