It is a known phenomenon in Russia, which is worth mentioning for the curios. In the 90s, we had a surgency of a Tolkienist larper movement that the normal people simply called "Tolchki" (derogatory term for a toilet). It all began with larping in the forests, innocently enough. But it didn't stop there.
At first, they pretended to be Legolas or Mithrandir and ran amok in the woods, but in 1994 some of them went further. They didn't exactly become insane all of a sudden, not yet. They started doing something that they called "catching short hallucinations" or "catching glitches" (gljukolovstvo or gljukonavty). It's a voluntary effect of an active imagination where a person deeply wants to believe that they experienced something magical when all they experienced was basically a big fat nothing. It may have been a random thought through free association, a dream, seeing some funny looking shadow or a cloud for a moment, but they'll attribute a deep meaning to it ("I SAW SAURON") and then convince themselves that it was real. Of course, at this point deep down they knew it was not.
But as with all forms of insanity, it was catching. When your best friend sees "auras" around people, you want to see them too! More and more people started pretending, and others were even able to convince themselves. Some of them went on to pretend to be "real mages".
But this wasn't the end of the road, yet.
Imagine being a gljukonavt yourself for a moment. It was one thing to be larping in the woods with some uggo Masha from Tverj. It's another thing entirely when Masha just "caught a glitch", in which she "recalled" being... none other than emperor Tiberius Caesar Augustus. And thus, another niche movement was born inside an already deranged one, and it was called "
vselentsy" or "inhabitors" (as in "someone who inhabits someone else's body"). Ah, an emperor! What an incredible luck to be in the presence of such a prominent figure when you're larping with Masha. So to not lag behind, you thought really hard and it turned out that you were none other than Vipsania, Tiberius' first wife! AND YOU BOTH REMEMBER EACH OTHER!! HOW CRAZY IS THAT WOW. Well, it makes sense in retrospect that kindred souls should find each other in their next lives, doesn't it!
So now you had people going around and claiming to be an isekai from another era. It didn't even have to be Tolkien related anymore, it became its own thing.
Most of them still don't believe deep down the delusions that they spout on the innocent and the sane, but it does deserve a quick visit to a psychotherapist. It's some kind of monumental coping with boring/bad reality at the very least, flagrantly enough.
Others will actually believe it for a period of time, but then admit it was a "passing phase" and become ashamed of it.
The rarest of them all will get a permanent mental disorder.
Our fellow codexer must have walked this path for a long time to become a vselenets, and it keeps walking in that direction. Who knows how far now...