I do actually know of a sort-of exception to this; Homura. In official media her witch has manifested with a few significant differences in different timelines;
In PMMM Rebellion, Homulilly manifests as the 'Nutcracker witch' with a Self-sufficient nature, while in PMMM portable if she should witch-out she manifests with the same name, but a different appearance and nature; She's the 'Mortal world witch' with a nature of Closed circuits. While the latter is obviously non-canon, many things from that game do seem to be, such as Opelia (Kyoko's witch) and Candeloro (Mami's) which lend it some weight.
The PMMM Portable witches actually
are canon, at least in the sense that they are what the girls would have witched out to in that point in time, given those circumstances. The "timeline" the game takes place in may not have officially happened in any of Homura's loops, but the witches themselves are indeed official, and I'm pretty sure were signed off on by the original creators of the series.
Also, Homura's witch changing isn't an exception — all evidence suggests that this can actually happen with
any magical girl. Witches do not change names so far as we're aware, but in appearance, type, nature, and even powers they can change quite a bit, depending on the cause of the magical girl's despair, as well as the circumstances under which they witch out. See: the alternate Oktavia (with alternate barrier, look, and familiars) seen in one of Homura's previous timelines, the fact that Candeloro in The Different Story manga has Sayaka-inspired familiars (rather than the Madoka-inspired ones seen in PMMM Portable) because Mami grew closer to Sayaka in that timeline, literally every "holiday" doppel from Magia Record, etc.
Homura's witch simply has the most drastic transformation out of all of them, because not only has the most time passed between her potential witch out moments, but the literal cause of her despair changed over time (I can't save Madoka no matter what I do --> Madoka saved everyone, but left me here all alone, and I 'know' now that she wouldn't do it again if she'd known what would happen). Hell, we can literally
see how her witch progressively changed over time. The Homura from early on in her quest to save Madoka (when she still had the braids, and was still optimistic) would have become the original
"mortal world" version of Homulilly, while by the time Homura lets her hair down and becomes jaded, the circumstances have changed enough that while her witch's type remains the same,
her appearance, nature, and powers have not. You can even see the dress beginning to form, and the Clara Dolls starting to extrude from her (which, interestingly, have not taken on individual appearances yet, even though they most definitely are said dolls according to Homulilly's doppel description). Finally, by the time Rebellion rolls around, Homura's circumstances and source of despair have both changed completely, and thus her witch
becomes something almost entirely divorced from what it was originally, sans the lingering symbol of the witch/record hat, and
with a second form to boot. The Clara Dolls are still around,
but have become familiars, separate entities to the witch herself, and her barrier is nothing like it presumably would have been initially. Thus Homulilly's evolution into her "new" state is complete.
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