There is an issue with the Denoiser, When rendering VR cube maps (also the other formats but to a lesser degree)
The edges between the images are also smothed. So that you get a fine edge where the maps are joining. witch makes it pratically useless, unlees you want to manually fix in PS. And for animation its hopeless.
You can offcause just render without denoise. And suffer death by samples:-)
The only way i see is if The denoiser recoqnize the format, and denoise with each images masked x6
Its a small thing. But it means your left without denoise option on those huge 12000x2000 renders.
Around13 hours on a Threadripper vs 72+ hours without denoise pr image.
So we're talking about the Arnold denoiser?
Are you running it on the command line, or as an imager?
My first test, with the Noice imager. The problem here is that edges are not denoised.