I can’t share an example file because it’s client work, but when I have exported IDML files recently (to fix a problematic/crashy INDD file), they always have 90%+ of the illustrations — including anything that is present on a master page — set to 0% opacity.

Fortunately, using the Find & Replace panel to search for anything with 0% opacity is pretty easy, but … it is a little disconcerting that things have changed at all!

(I’m on a Mac running macOS 15.2 and InDesign 20.1, but I had this issue with the previous version of InDesign as well.)