I have to disagree, from my experiments Photos does do a conversion from ProPhoto to sRGB, but does not use the monitor profile thereafter.
I set my BenQ to sRGB mode, i.e. it's native response is sRGB with no monitor profile involved. I then used an image rendered in ProPhoto, with a ProPhoto profile embedded, and viewed in Photos, it was displayed correctly. I took the same image and viewed in Paint, there was obviously no profile conversion since the image came out in that dull almost colorless state that is characteristic of viewing a ProPhoto image on a standard (sRGB ) monitor with no conversion.
I believe it you google the subject you will find numerous posts supporting my conclusion wrt to Windows 10 semi-color management.
When you say "it's native response is sRGB with no monitor profile involved", what do you mean? Did you have your monitor set to sRGB mode, and have a monitor profile set that corresponds to sRGB mode?
If there's no monitor profile (or a monitor profile that doesn't correspond to the monitor's current state), then colour managed isn't going to work anyway, and colours are likely to be wrong in most circumstances.
In my tests, using a monitor with sRGB gamut correctly calibrated/profiled (i.e. with a profile that reflects the monitor's sRGB gamut), the Photos app certainly did not display a ProPhoto RGB image correctly.
In addition to the images I linked above (in sRGB and Adobe RGB), here is the same image but converted to ProPhoto RGB and saved with an embedded ProPhoto RGB profile, in order to replicate what you described:ProPhoto RGB version
When I view this in Windows Photo Viewer (or any other colour-managed viewer, such as Photoshop) it looks the same on a wide-gamut monitor and on a standard-gamut monitor (both calibrated/profiled).
However, if I view it with the Photos app, it looks grossly under-saturated on a (calibrated and profiled) sRGB monitor. It also looked somewhat undersaturated on a wide-gamut monitor, which is again what one would expect.
Unless we're somehow talking at cross purposes, I don't understand what you have found.
PS - "I believe it you google the subject you will find numerous posts supporting my conclusion wrt to Windows 10 semi-color management."
Can you find me any links that say that? I'm not disputing what you say, but I've googled, and all those I've found with a view on the matter suggest that it is not even partly colour managed!
Are you thinking of Edge and IE? They are both partly colour managed: they ignore the monitor profile, and always convert the image to sRGB.