If you no longer have the sensation of it moving freely around, I think your brain will get used to that and adapt; and also it's not behaving like a penis anymore because of the HRT.
The opposite happened for me, what's there started behaving more like a penis and doesn't particularly feel or operate like a vagina. Hormones defintiely have something to do with all of that and it takes the brain a few months to catch up but it does.