Thumbnail sizes not changing even after creating new thumbnails

I apologize for resorting to posting here: I’ve read what I could find related to this issue, but nothing did the trick for me. I disabled my custom CSS, too, with no results. Here’s what I’ve tried, in detail:

1) I’m kinda-sorta using Gutenberg, but I have to use the “Classic Editor” to generate the intial gallery shortcode (I then can edit that in Gutenberg).
2) Across multiple galleries, I’ve ensured I have Override thumbnail settings UNchecked and I’ve changed my default thumbnail size to 220×165.
3) I clear my browser’s image/data cache and refresh the page (Shift + F6 [Windows 10]).
4) I revisit the page in question and…the thumbnails are the same 240×180 they were when I started. Except on another page, where they’re 200×150 (where I initially overrode the thumbnail size, but since removed the override setting)…

So my question is: why aren’t my thumbnail sizes updating? I considered deleting my galleries entirely and attempting to recreate them from the ground up, but surely there has to be a better solution? I’d appreciate any advice. Thank you!

The page I need help with: [log in to see the link]