You may think it is bizarre that they would pack a box of mainly plastic bags but what are they supposed to assume? If they don't pack it what are you going to do with it? And whatever it is, why haven't you done it beforehand or moved the box somewhere out of the way?
My plastic bag remark was meant as an add on to the lighthearted remarks of weird things that we've had movers pack, most people expect a few things not thought about to be packed, but if we want to get serious about it:
They weren't to assume anything - we gave them detailed instructions by the video tour and my spouse gave them further instructions when they came around though we were under the impression the latter was just a refresh of the former due to COVID. When I checked in with them I was told they had all the information they need, afterwards when we complained we were told that my spouse was were too vague and had left too quickly on the day. That's why I suggested people stick around more than they think may be needed, label more than you think you should or they say is needed. Our following the advice to just get out of their way bit us in the ass.
We'd tried to have things not being moved out of the way - we had a whole room they weren't meant to go in, plus another they were only meant to take the exercise equipment placed by the door (that instruction was entirely ignored) and a third where a large storage unit had been (that we'd disassembled and moved ourselves) which had a pile of things to take mostly already boxed and a pile of things not to take.
The latter is where the plastic bags were - not in a box, but a couple bags that had more bags in them. We were going to use them afterwards as part of cleaning out the rental and then take those unused to the store for recycling. Many of the bits, like my sons' bunk bed frames, we couldn't really move out of the way. We were told that was all fine. Afterwards we were told the place was too much of a jumble and they took photos of the discard piles in the room we'd put to be out of the way that they were told was going out via house clearance as evidence of our poor hygiene and threatened to contact social services if we didn't take down a negative review.
My movers chose not to move a bookcase, an upholstered chair, 2 mattresses, some of my bed slats, a walking stick I found in the front garden, and several other items so I think I can have a chuckle that they did choose to pack plastic bags and a 95% empty fizz bottle. People who had better moves are allowed to have a chuckle about cigarette trays with fag ends in them, nappy bins, or their own silliness with passports - it doesn't mean any of us don't recognize what a big job packing up a whole house is, that's kinda why we'd got people in to help.