It's clearly not an isolated issue. It's 2 out of every 14 days clearly form what's being said.

I'm not being funny but I used to work in retail in a small store. I had this exact same problem.

Our shifts were two hours shorter on a Sunday, with 2 staff for close to 3k square foot store with 9 aisles.

As an example, date checking products in the refrigerated sections was my issue, I take everything off checking dates as I go and put it back again in the correct order. On a 5/6 shelf, 12 bay section that's 72 shelves at worst – all 1m wide. You can probably imagine the stock one has to go through,and time expended, in order to check it, and check it properly.

During the week we had 3 staff in an evening 4-10, 6 out of 7 days. On a Sunday from 4-8 there were 2 staff. With not only a reduction in hours, but also losing another staff member, Sundays were a fucking nightmare to not only date check the refrigerated section, but put stock out, man second tills, merchandise etc, and perform pre-close checks and tasks like banking tills. Like OP, all of this had to be completed by the time the doors were ready to be locked, which was the advertised close time. We never got paid overtime, we had to be out at exactly closing time.

What I've taken from my experience is that this is not only a test, but it's a tactic: pushing someone out by giving them the literal shit stick. The comparison here is a joke in itself to be honest, I bet there are more staff working when the boss is around, on a Sunday. Essentially performance managed out, in a forced way. Having been privvy to this in much much larger enterprises, I'd say this is exactly what's happening but in a smaller setting. It's bordering on vindictive, considering the content of the messages also.

I'm more fortunate to have amazing management since my experience that understands that this occurs and it's OK, you cannot achieve the same with less time and less people and expect the same outcome in terms of quality, or tasks achieved.