I'll tell you a true story (names changed):
Years ago, my dad and my little brother were working together, mending a bicycle.
My dad said something like "pass me the spanner, Martin".
Martin wasn't my brother's name, it was the name of my father's youngest brother.
It wasn't intentional and certainly not malicious. My dad wasn't suffering from dementia, nor was he (so far as I know) ND, or in the habit of muddling names.
His theory was that some part of his mind had reverted to when he used to fix bikes etc. with his little brother.
That may well explain that particular incident, but there's a wider implication, which is that if we're focussing closely on the matter in hand, the part of our mind dealing with routine matters of language is more or less on autopilot. What it retrieves for use may be the latest, corrected and updated version of the information it needs, or it may be some earlier version that's still kicking around in the brain. In this case it managed to conflate two different people, let alone pre- and post- transition versions of the same one. Without intent, without malice, and without any disability involved.