Hi,
HIGH PORTB.0 works because it takes longer to execute than PORTB.0=1. This is because it also sets the TRIS register each time they executes.
In this case the extra delay introduced by the inherit write to TRIS was enough to allow the voltage on the pin to rise above the threshold within time to avoid the RMW issue. But there's no guarantee it'll work the next time as it depends on the oscillator speed (as you've noticed), load on the pin, PCB trace capacitance and so on. For devices which have both a PORT and a LAT register you really should write to LAT to avoid these kind of RMW issues.

Good luck!
/Henrik