Welcome Jeff,

The rotor housings looked great.  How does the coolant get into
the rotor housings once the engine is pulled? Unless the coolant is poured, or spilled into the ports, it shouldn't be able to get into the rotor housings.  If the combustion O-ring was bad, coolant could get from the cooling passages into the rotor housing.  Take a close look at the O-ring grooves on all the side housings.  Was it in both housings, or just one?   Was the intake manifold on the engine when you got it?  If not, maybe someone just got careless when they removed it, and managed to dump some coolant in the engine.   I remember a month or so
ago people talking about newly rebuilt engines doing this.  How does
that happen?

Not exactly.  We were talking about coolant in the coolant passages.  If this sits long enough, it will rust the cooling passages.  It shouldn't be in the rotor housing though.