Yes, HDR10+ also works again with the internal Prime App on my 55OLED804.
@Kelvin: when starting with HDR-Content (e.g. Jack Ryan), my OLED804 presents the message “HDR10+” right below for a few secondes, and in addition you can deal with the HDR-Options in the picture-menu again.
I also had to update the Prime App (and in addition I found somemore updates for system apps, which were presented automatically in the App Update Menu). Reason was, that I had downgraded the Amazon App to its factory status before.
I also can confirm, that Dolby Atmos won’t work with the internal Amazon Prime App (Jack Ryan, english lanugage version), up to now only with an external streaming-device: in my case with the chromcast Ultra 4 (with “Google TV”).
(No problem for me, because I don’t use a external sound bar).
Netflix Streams (internal app) have continuously worked in Dolby Vision and in Dolby Atmos, also with des short message “Dolby Vision and Dolby Atmos right below.
Mayby you have canceled the message in the picture menu of your TV?