I recently played an HDR10+ file from a USB stick and can confirm my 2017 MUseries TV recognised it as HDR10+ but could only see the standard HDR tag. I don't have an Amazon stick to test.
Hope this helps.
Indeed the update @Barbuslex has posted is for the NU series and from the French site, it is not yet listed in the Uk one.
1270.3 is the latest firmware for the MU series to date.
Why you think tour tv recognized that file as HDR 10+ if it displayed it as plain HDR 10 what is the clue? I think tour TV took the HDR 10+ file and downgraded it to HDR 10 to display it.
An HDR 10+ Tag is what se should look for. That is the only way to be sure. It would be pointless to input HDR 10+ sources if the tv is not capable to process them and downgrades everything to regular HDR.