preaching, stealth and sieging forts

[Tuidjy]

In my experience, it depends on whether the unit was already stealth-preaching
before your army besieged the fort. I do not think that an unit that starts
preaching in a province owned by you is considered hidden.

[Endoperez]

I seem to recall pressing "preach" and first pressing "hide", then "preach" have different effects. The first would have you just preaching, the later stealth-preaching. I haven't tried this myself, but it could work.