If you're interested...
If you use a plugin with a higher inherent latency than the block size you set for the system, Reaper adds additional blocks of latency to keep everything in sync. And that's how you "break" your latency setting with a plugin like that.
To use a 3rd party control panel (like the proprietary RME control panel) to control the sample rate or block size on a Mac, untick the box next to either parameter on the Reaper Preferences/Audio/Device page to disable control from Reaper.
SOP is to control sample rate and block size from the DAW control panel. Only turn to OS audio utilities or 3rd party control panels if a particular device requires it or there's some other faux pas forcing the issue. Reaper being Reaper has your back with the options to do just that.