@Gilles This works! Thanks for the tip; I'll go ahead and purge as many PNGs as I can!

@Aklapper Firefox Nightly 63.0, Windows 10 version 1803, Ctrl+F5 doesn't work either; I've been trying on the Microsoft Excel English Wikipedia article to get the lead image to show up with a transparent layer with all three aforementioned methods.

@Gilles I should also clarify, just as I had already said earlier, the opaque backgrounds still appear even after my browser's cache was cleared.

@Gilles can the everyday Wikipedia user such as myself help in the process of purging these PNGs? The usual "?action=purge" URL technique doesn't seem to work.

@Ahecht it seems to be fixing itself... rather slowly. Most of the files I've uploaded when the transprency layer bug was around now have their transparency layers in thumbnails, but some still have the opaque background. For example, the lead images in the Microsoft Office 2019 and Microsoft Word articles on the English Wikipedia now appear as intended, but the image on the Microsoft Excel article still has an opaque background; my browser cache has been cleared and both the article and file page have been purged. Though, I suspect this will be fixed naturally as the server catches up with all these files.

@Aklapper I wouldn't know how to fix this, I'm just a Wikipedia editor that was pointed to this task by fellow editors about a month ago. I'm simply trying to express the most amount of frustration with the least amount of repercussions towards the fact that even after two months, 30 subscribers to the task, and potentially hundreds broken PNG files strewn about Wikimedia, work towards fixing this relatively major flaw seems non-existent. I'm sorry if I crossed a line or two, but I feel like two months of sitting on something this major is too long. If there's a better way to express frustration and prompt more immediate action in situations like this, be sure to let me know.

This is a fundamental flaw in the image system. It still hasn't been fixed, and in the interest of having properly-rendered PNGs on Wikimedia after almost two months without such, it really needs to be fixed soon... :/

This image successfully retains the transparent background layer when previewed on its File: page, but when used in the article it was designated for, the transparent layer still does not appear, and it is still a solid white.