Absolutely agree there; my assumption was that this was changed to make the text more easily visible, which I figured fell under the accessibility options.
Confirmed to affect 1.19.4 Pre-release 1 (as of 02/22/23)
I would imagine they don't need it to include non-UI elements. If you pare it down to just the main folder structure and the contents of the GUI folder, that would probably be enough. Would also highly recommend putting it in a zip file rather than uploading the folder itself, as it's very hard to parse what's what out of a jumbled list of every file in the pack.
Seconding this for my pack. Working through snapshot-by-snapshot, this appears to have begun in 23w05a, and has continued into 23w06a and 23w07a. As best I can tell it only affects buttons using the widgets.png file. If necessary, I can also provide a resource pack example to demonstrate. The same pack worked fine in prior snapshots and releases (tested from 1.19.2 all the way through 23w07a, with only 05a, 06a, and 07a presenting any issues).
Still seeing this with paintings and beacon beams in 20w17a.
Paintings confirmed to still lack transparency in release 1.15.2.
I'm having a very similar issue, though unrelated to structures. They seem to be spawning in hordes as of the new snapshot, though I'm not sure whether or not that has to do with spawning over time or if they all spawn at once.
I've attached an example pack, apologies for that!