I don't know exactly what to make of this, but encountered this situation for the first time myself.

Booked intra-EU for 2, waitlisted both pax for both flights. Both flights U1, and that seat also immediately claimable. For me to get it, however, would require splitting the booking (no issue with this, makes sense).

However, enquired Plat whatsapp on the procedure in these cases. According to them, that U1 can be claimed by anyone travelling solo, despite ppl already on the waitlist. So, booking group size seems to be a deciding factor vs. your status. I.e. if there are 5 groups of >1 with Lumos in them, their upgrades wouldn't go through if U never reaches their party size, while the common folk can take up all the U1 that AY decides to release one at a time.

I presume this is not a problem yet in most cases, as U availability is anyways limited and tends not to increase despite seats being available, but if AY actually moves to a bit more dynamic and intelligent system with U availability this could cause problems.

This might explain why I have never had an upgrade not clear, since I've practically always travelled alone. Sorry to all Lumos travelling with family

Not sure what to make of this, but thought it an interesting data point.

Edit: Kudos to AY, my upgrades went through after asking about it. However, agent had to open issue with "Plussa osasto", i.e. the infamous "other department". Adds to the recent finding that agents aren't empowered to do very much themselves. Plenty of space still, I9, X9 with a big bird on euro route