It is indeed normal for most SP games to lose majority of its audience six months post release. However, as pointed out, the player numbers are not normal for a Bethesda RPG. Fallout 4 and Skyrim have quite a bit higher concurrent numbers despite being much older games.
The main issue being of course that Starfield is disjointed vs their previous games. There is no “what’s over there” moments really where you can organically discover interesting quests, landmarks and so on.
The game overall seems to also have been written even more inconsistently, with generally more boring story and generic/bad characters. How they pulled it off after Fallout 4 Institute nonsense is beyond me.
Then you have still no proper mod support, modders in general being less interested in the game, and very slow level of patching. Compare what BG3 team produced in roughly the same period as what Starfield did. It’s night and day. I am guessing that instead of patching they are furiously working on DLC… which isn’t a great look.
There is more of course, but that’s why Starfield has fairly low concurrent numbers as compared to their previous games, IMO. It really should be quite a bit ahead, but the game is just isn’t good enough for it to be a platform for 10 year support or whatever. And unlike Cyberpunk it would be very hard to improve Starfield due to its design.