Lightroom ignores most camera settings for RAW files. If you want the camera settings shoot JPG.
Well, at least the hi-iso NR setting of RAW file from the good old Nikon D300 is in effect in the same version of LR 6.51.
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The camera LCD and IrfanView are showing you the camera-processed JPG that is embedded in the raw file for quick viewing.
Nikon-rendering is raw data plus Nikon-settings from the camera or using Nikon software. You can change Nikon settings in the camera or in Nikon software. You cannot change Nikon settings in Adobe software.
Adobe-rendering is raw data plus Adobe-settings using Adobe software. You can change Adobe settings in Adobe software but not in-camera settings or in Nikon-software.
Where are you seeing that the high-ISO NR setting from the camera is affecting Adobe rendering?
It's possible that the D300 is actually doing something to the raw data before writing it to the raw file--Nikon is infamous for doing this, so both Nikon and Adobe processing would start with that.
It's possible that the D500 doesn't modify the raw data before writing it to the raw file, anymore, or at least you have the NR set not to do this.
Or it's possible that you've customized your LR defaults per ISO for the D300 and haven't yet done that for the D500. If you are seeing different LR NR settings for different ISO photos as they're imported, then customized-LR-defaults-per-ISO seems likely.
Some clarifications -
I just double checked. Actually even for D300 RAW file, the Nikon in-camera hi-iso NR setting is ignored by Adobe.
That makes sense.