Spawning in 1.16.5 time wise is different alongside BYG and has lead to spawn rates for legends and bosses being weird. Not spawning as frequently or spawning more frequently due to spawning changes. Underground spawning areas for Regigigas or other Regis do not work as intended as you need open air above them.

As discussed in the Discord, Bosses and Legends both spawn on a timer, and have no set intervals. If there is a discrepancy, we need more details. How is the time different? How do you know they are spawning more or less frequently?

I tested spawning a Regigigas earlier, and it spawned in Underground areas just fine. Had to dig one out of a gravel pit too.