In which case I'm seeing what you are seeing.

However, digging a bit deeper suggests to me that this is indeed either a glitch or something that has not yet been properly finalised and filed. Looking at the timetable filing (via ExpertFlyer), what I see is this:

On Thursdays, BA792 is a 320 from 29 March (the first Thursday in the northern summer season) to 26 April.
BA792 is a 321 on 3 May only.
BA792 is a 320 again from 10 May to 7 June (at least).

In contrast, BA793 is listed as a 320 for all the Thursdays from 29 March to 7 June (although that line is actually published as days .2345.. from Tuesday 27 March to Wednesday 13 June).

So my suspicion is that someone thinks that a 321 will be needed for LHR-GOT on 3 May for some reason, but hasn't yet adjusted the GOT-LHR allocation for that day.

IME, schedules will typically get finalised for the northern summer season during Jan and Feb 2018, so I would expect to see this kink ironed out at that stage. If it isn't, and you're still worried at that stage, then you might want to think about contingency plans.