After some exchanges with Phase One support (I even sent them a screen recording), they say that I am doing everything right, they are creating a bug report / feature request to have this improved for a sharper preview from the raw.
First, thanks a lot for creating a detailed support case.
Second, don't get your hopes too high. This issue has existed at least since version 9.
I created a respective support case a long time ago with screenshots, exported images, etc.
I had some dialogue with the support team and appreciate the steps that have been taken (e.g., the Proofing option), but a proper solution is still missing.
I am rather certain that this "Fit" view deficiency is not regarded as a bug by Phase One. I believe they regard the solution as a compromise between accuracy and efficiency that is "good enough". Apparently it is good enough indeed for many because I'm not seeing mass protests. I doubt that the non-complainers see a different results, I believe they just don't have the same expectation towards a "Fit" preview.
The fact that this basic requirement (useful renderings at "Fit" size) has not been addressed for version 12 and efforts have instead been spent on changes/additions that almost seem obscure in comparison, does not instil confidence that we'll see a fix anytime soon, AFAIC.
I can only encourage everyone who notices how blurry "Fit" renderings can be at times (not due to bug, not to C1 needing a restart, just the regular lack of crispness which contrasts to the rendering at magnification levels 30% or higher) to create a support case. I believe Phase One should have ample detailed support cases that include evidence already that a quick note should be sufficient perhaps with a pointer to this thread.
Here's to hoping that the issue will be fixed some day. I realise that an optimal solution (which has to take efficiency in both preview generation times and preview storage into account) is not trivial, but the current situation could be easily improved by taking the user's standard preview size into account and optimise generation for that instead of one of the standard preview sizes made available by C1.
Personally, I don't see myself upgrading before the issue has been fixed. Yes, having radial masks, etc. would be nice, but I won't spend the substantial upgrade fee to be left with the same preview issues as before.
P.S.: I'm experiencing the issue on a Windows machine. The fact that you are seeing it on a Mac corroborates the notion that this is not a bug but rather a consequence of how "Fit" previews are handled by C1.