This seems to happen pretty often, I haven't watched back any fights to see what may be the cause of it, but it happens to me quite often in endless archive. It also happens in PvP and I imagine every other part of the game where you hold block and encounter enemies, it's just extremely obvious in endless archive because of the sheer amount of time I hold block. It is similar to holding M1 or left click when heavy attacking and having your character doing nothing, holding M2 will at some points just stop holding block and the only way to begin blocking again is by releasing M2 and repressing it.