FYI - for anyone looking for 4K content on Max, searching for "UHD" or "4K" in AS or even on Max web site only returns a small number of titles. I found this page from HD Report with a list of all of the 4K/UHD content on Max. So far all of the titles that I'm interested in, are indeed in 4K, but I can't guarantee how up to date this list is.
Tip: Click column headings in the page below to sort by New status, by Category (Series, Doc, movie), HDR type, etc.
Code:
https://hd-report.com/list-of-max-4k-hdr-atmos-movies-shows