Roku Ultra 2020 4800X running 11.0.0 4193-C2 plays UHD and all HDR formats including Dolby Vision, as shown here playing Thor: Ragnarok on Disney+:
Lord of the Rings: The Rings of Power on Prime Video indicates that it has Dolby Vision (which is only available with UHD, unlike HDR10, HDR10+ and HLG)
but when played only streams in HDR10+
This behaviour can be replicated with other titles which have played on the same hardware with Dolby Vision such as Amazon's Jack Ryan.
The Amazing Spider-Man 2 is also available in UHD with Dolby Vision
but plays in HDR10+, not Dolby Vision.
I am uncomfortably reminded of when HBO MAX silently disabled Dolby Vision for a long time with neither the application owner or Roku providing any useful information to their customers, but am optimistic that this issue can be identified and resolved promptly.
Updated after posting to remove issue which resulted from rental being HD, not UHD.