I have tested the same scene in 20 and 20.5 rendering with Mantra and adding a cryptomatte object and material output.
The exr from 20.5 doesn't load the channel data in COPs or Nuke. The same scene rendered in 20.0 and below loads the cryptomatte data perfectly fine in Nuke and COPs. Is this a known issue? Is there a new option needed for 20.5 Mantra renders? Any info on this subject would be greatly appreciated.

Indeed there seems to be a bug currently where mantra's cryptomatte metadata isn't being written out to header when using the new OpenImageIO EXR driver (which is 20.5's default).
I'll log a bug, but in the meanwhile, you can work around this issue by setting the environment variable "HOUDINI_OIIO_EXR" to 0.

Thanks so much.

Just a head's up that this should be fixed in tomorrow's daily build of H20.5