You’re definitely not alone, I do this too
and what’s worse, my co creator does this as well so we just sit there together, working on the plot and then like:
Me: “oh right, gotta give Doyun a surname though. Tanya also doesn’t have one yet… and Arseny… also Mina’s grandma…”
Them: “yeah, right. Gotta do that.”
Both of us: proceed to talk about irrelevant stuff and forget about this again
Though to be honest, there’s two things worse than this, like 1. Me not even thinking that a last name might be required and then struggling a lot to make one up from scratch (this doesn’t sound like a problem, but trust me, as soon as it comes to royal families it is) and 2. the thing my friend and co creator does a lot, which is giving the OC a surname only and then spend 2 hours trying to come up with a name for them, only to discard it 3 days later because it sounds like the name of some controversial historical figure. Mind you, this exact thing happened to us twice. I must really suck at names lol