I found it!!
Okay, I could have saved myself a lot of trouble by just going to the Wiktionary page for 仙 and seeing that your mystery character is listed as a derived character, but that's hindsight speaking.
I thought about it a bit, and I realized that if you were able to see it rendered, there's no way it was actually a PUA character, so that was a red herring. After unsuccessfully searching in a few online dictionaries, it dawned on me that I could just look at the master list of Unicode characters. Mind you, there are nearly ONE HUNDRED THOUSAND CJK characters in Unicode, so I couldn't exactly skim over them. Luckily, on the Unihan Database Lookup page they provide a helpful radical search tool. I rarely search by radicals, so I was a bit confused when I clicked on 3-stroke radicals and couldn't find the water one used in the character, 氵—turns out that abbreviated radicals are still categorized with the stroke count of the original radical, which in this case is 4 strokes for 水. Once I figured that out, though, all I had to was select the radical, set the minimum and maximum additional strokes to 5 (since that's how many strokes are in the non-radical component, 仙), and then scan through the ~200 characters in the results. And...bingo!
𣳈
I'm gonna be real—I still don't really know what this character is. From this page and this page I was able to learn that it's part of the Hong Kong Character Supplementary Set (and this character in particular is part of the Unicode CJK Unified Ideographs Extension B), and some pages only gave a Cantonese reading for it, so if I had to guess, it's probably part of a place name or used in personal names in Hong Kong (or the Cantonese-speaking regions of China more broadly). Seems to show up a lot as part of the two-character compound (?) 潮𣳈, but it's hard for me to understand more than that not speaking a lick of Chinese.
Also...none of this explains how or why a Japanese person would randomly produce this character with a standard Japanese IME. Still wish I could solve that mystery.
edit: here's a bunch of sentences in which the character appears...again, gonna need a Chinese speaker to interpret this
I actually did consider an encoding/decoding error as a possibility! I remember like 10 years ago my phone had a bug where all my text messages were showing up as garbled Chinese characters. Not sure why that particular emoji would become this random character, though. I doubt you're as interested in this as I am, but if you wanted to investigate a bit more you could download another text messaging app (e.g. QUIK SMS on Android) and see if it renders the text message any differently.
I'm not disappointed—I love going down these little research rabbit holes! I learned a few new facts and discovered some useful resources that I can employ in the future, so it was well worth the detour.