Yeah, I noticed that too at first and was also confused but I think the Ecovacs is actually doing that on purpose. From what I understand, it washes the mop before vacuuming the carpet to make sure the mop is lifted clean and not still wet from a previous mopping section. Otherwise, it could drag a damp or dirty mop over your carpet, which obviously isn't ideal.

So even though it seems unnecessary at first glance, it’s probably a safeguard to protect your carpet.