P: Camera Raw: Canon R5 Raw images are underexposed

Lisa, Denis’s point above about careful setting in the Details (and NR/masking) settings may help too, your in camera Preview will have in camera processing applied too, which might point to your noise issues.

Adobe’s default NR and detail settings are far less than ideal for most uses, even if they are scientifically more accurate. Tackling the currently rather unfriendly method of setting up an ISO variable preset is worth the effort.

I don't need Lisas raw to understand, perhaps unlike you, that raw rendering has no effect on the raw data. It is utterly non destructive and raw is READ ONLY.

Thanks Andrew, I did not realise you had been able to look at this bug with a Lisa's R5 file yourself to check that there was no loss of image data in the rendering from ACR.

My take on it had been - Using your analogue analogy, the potentially perfectly exposed negative (the raw file) may indeed have had all the shadow detail and black in the right place, but if the "development chemistry" is done incorrectly (the bug in ACR)  there may result a very dark image.

This could be processed to look ok (boosting the "exposure" in ACR)  but may not be optimum in terms of noise level, hence the OP finding a noisier image than they were expecting.     

I am not saying you are not correct that no data has been lost I am just saying that it could potentially be the case- if black was being translated .8 of a stop blacker than it should be. 
That would quite easily be measured of course.

The loss exists with or without your edit. Because you under exposed the data. 
Analogy: You have a perfectly exposed negative and you place it into an enlarger. The ideal exposure for the print paper is at X seconds at F5.6. You set the enlarger at the same time but at F8 and the print is too dark. That has zero effect on the perfectly exposed negative. 
Another analogy. You have a perfectly processed digital image that appears as you desire. You turn down the brightness of your display, it looks too dim. That alteration of the display had zero effect on your digital image.

Raw is raw. This bug has ZERO effect on that image quality. The bug is as if you opened an image with defaults and the " Exposure" Slider is set to minus 1 stop. The bug shouldn't show that but it does. You alter the " Exposure" slider so it doesn't look one stop too dark. It  had ZERO effect on the raw data. IF doing so makes the image look worse in the shadows, that's utterly due to your under exposure of the raw data in the first place; the raw wasn't affected one bit by the move of the slider. 

The bug is a bug that should be fixed but the bug has no effect on image quality of the raw. That was all your doing when you actually exposed the raw and exposure is solely an effect at capture based on shutter and aperture. 

The rendering is not stretching the shadows or shifting blacks, causing data loss or degradation but it is showing you that you didn't ideally expose the data. The rendering can't alter the original raw data. 

If the rendering is stretching the shadows/ shifting the blacks of the exposure data that are in the raw that would change the noise floor. 

Hold out for a fix from Adobe on the profile sometime, don't give up on the raws or in the meantime Capture One (13.1.2) now supports the R5.

I am finding the quality loss to be in the dark that is being lightened, it gets a bit grainy.... I did late night sunset shots that were amazing in camera, but suck on my computer and I've never had this happen until now 😞 

No deterioration; raw is raw. Now if you under exposed yes. But the wrong rendering is the bug; no effect on the data prior to this. You adjust the rendering and you get a better appearing image and the data is what it is before and after. 

sure hope the fix is coming soon!  images look so good in camera but are so dark and lack color when downloaded, and fixing in post cases some quality deterioration...  😞 

And better news is the bug has no effect on the raw data itself. Make a temporary preset until the fix.

Adobe has acknowledged this as a bug.  They are working on it.  People need to understand that Canon does not cooperate with Adobe in any way, at least so far as I know.  It takes time to reverse engineer the RAW file format.