It seems that if you leave in the middle of sweeping (or start of after reset), the sweep option gets nuked.

I had a simpler case that all happened in one day, where I reset at noon, sweep 1-3, log off, a few hours later in the same day(around 5-6pm local AND server time), come back, sweep 4 and notice that 5-9 aren't sweepable. After manually clearing everything, I was able to sweep 1-9 again the next day. To the best of my knowledge, there was no maint during the time I was off (I play on LA time server, no maint in the afternoons)

I would guess that whehter you can sweep a node is a boolean that is checked and generated at the moment of reset. The boolean reverts to default, which is false, if it is left alone for too long or something.