I'm using this photo in Raw and jpeg as a "guinea pig" to learn DPP4 and PSE 2021. I have played with DPP and PSE Quick to get a decent photo. I want to soften the glare on the rhododenron leaves in the background. Your suggestions are most welcome. Adding- had to reduce it to fit.
This is how I do things and others might not like it, but I hope someone might find it helpful.
In DPP, with "Show highlight warnings" threshold set to 255, it is not true that there is no data in the parts that display as red in the RAW file, but only in the color mapped result. It also may be true that some color channels are clipped in parts that do not display as red. This image was not quite as extreme as yours, but I wanted to recover parts of the red channel that were over exposed. By sliding the bar on the right side of the "Gamma adjustment" histogram display to the right, the blown out red channel was displayed more like I wanted. The cost, is that the image becomes more like an HDR image which had been squashed into 8 bits per channel.
I also moved the higlights slider to the left to make the highlights less bright. The combination of moving shadow to the right and highlights to the left reduces the dynamic range of the image when I have captured an image with too much dynamic range and want less.
When increasing the dynamic range by moving the right side "Gamma adjustment" in the histogram, it is possible that the over exposed regions might change from white to unwanted colors.
Giant Swallowtail (Papilio cresphontes) on Zinnia, August 13, 2021
I pulled up a recent photo of a cypress grove with the foreground very light. Bright morning sun was over my left shoulder while standing on the road. clicked on the Higllights icon but can't get the Gamma adjustment "activated." The lower part with Highlights isn't "lite up" where it can be used. The Auto button isn't working. Any help is appreciated. Got the general idea but can't do what Johnr is suggesting.