For which reasons might a UK immigration officer refuse entry at the border with a valid visa? Are there stricter criteria during the pandemic?

My significant other has just been granted a 6 month multiple entry visitor visa to the UK. They visited the UK twice before in 2019 with another visitor visa.

I haven't seen them for 14 months due to the pandemic; the purpose of the trip is to visit me

A couple of months ago they got covid-19 (so they now have antibodies against it, which makes them more willing to take the risks inherent in travel). I'm sponsoring the accommodation and other costs for the visit (we documented this in the visa application). Obviously due to the covid-19 restrictions, we haven't planned activities more elaborate than an outdoor walk (after the quarantine period, obviously).

Besides that, it seems to be a good moment for a visit (due to the decreased number of cases). There's no guarantee that a few months down the line (especially after restrictions are relaxed) the numbers won't get worse again, or Russia might be placed in the red list (preventing them from visiting at all).

From the UK side, travel from Russia is allowed for visa holders, with the quarantine and testing restrictions in place since February (as long as Russia isn't added to the red list). Confusingly, on the Russian side, there are reports of flights being suspended between UK and Russia:

https://ria.ru/20210316/aviasoobschenie-1601472354.html

and yet, flights between Moscow and London are ongoing:

https://www.flightstats.com/v2/flight-tracker/SU/2578

When trying to confirm the flight situation with the airline, they raised the concern that my SO might not be allowed in with their visa.

Obviously, a visa doesn't guarantee entry, and immigration officers might refuse it. But the specific concern in this case seems to be the reason for the visit, and the fact that it's a short term visa. I think this point is moot, since once granted, as long as the visit is complying with the conditions of the visa, the UK doesn't care about differences between visas.

The differences matter later (when renewing/applying for other visas, since some work visas might be able to be renewed, but a visitor visa instead is obviously limited to its duration and the 180 days maximum stay).

Since airlines might incur fines if the passenger is refused entry, I think it's more likely that Aeroflot might actually refuse boarding (rather than my SO being refused entry after landing). Before Brexit, I know that if their documents are valid, they could be eligible for compensation in such circumstances, though this is another matter.

I know that there had been refusals related to the pandemic, like in this case:

https://www.theguardian.com/uk-news/2020/apr/16/us-woman-refused-entry-uk-visit-pregnant-daughter

But there are at least two obvious differences:

• That was way earlier in the pandemic, while the rules weren't as clear (and thus the officers probably had leeway in making decisions). Compared to now, where there's requirement to have tests both before and after the flight, plus quarantining, which significantly reduce risks.
• That trip happened to a non-visa national travelling without a visa, afaik. Since my SO already got an entry clearance, I'd expect that there wouldn't be any surprising decisions.

In brief, I expect that the immigration officers will mainly try to verify that the purpose of the trip is allowed, and consistent with the entry permit. I checked the official guidance and it seems that all other conditions (deportation orders, criminal history, etc.) are either for much more severe circumstances, or circumstance that would've prevented the entry permit from being provided in the first place.

Is there anything that I'm missing? Should I expect stricter criteria than those listed in the guidelines?