Some stats (thanks TNT on the unofficial Wikimedia Discord for the links):
https://grafana.wikimedia.org/d/000000479/frontend-traffic?orgId=1&from=1631915084782&to=1631925884782&var-site=All&var-cache_type=text&var-cache_type=upload&var-status_type=1&var-status_type=2&var-status_type=3&var-status_type=4&viewPanel=7

OK, so for some reason the way the highway system parameter is being called is causing issues. If the highway system parameter is specified, it will break, but if I don't supply a parameter and let the Wikidata template find it (it looks for the ID for property P16) it will work. Interesting...

Yeah, it looks like the spaces for assigning a Wikidata ID for the highway system property (P16) to show was the issue. It must have caused a syntax issue with the stricter parser.

OK, I fixed the thumbnail issue by removing some trailing spaces:

The small empty map on the article is the issue - something is wrong with a parameter, so that small map doesn't get rendered while the full screen one is now properly rendered. That small map basically uses an image of what the map would normally display in order to cut down loading times and avoid accidentally moving the map window to focus elsewhere.

Weirdly enough, the pre-rendered map now works in my sandbox on English Wikipedia, but nowhere else with the highway systems template. Maybe something to do with the Wikidata ID entry? My sandbox page doesn't have a Wikidata ID, but the other pages I looked at did.

Yeah, even in HHVM the thumbnail doesn't render correctly (unless you are in editing mode, because there the thumbnail is actually a smaller dynmap and not an image). Still, thanks for fixing the dynmap.

@TheDJ Thank you for fixing the trailing commas. I did figure it was related to comma placement in some way, but I mostly focused my efforts on commas with spaces before them. I will look at reimplementation, especially since there is still an error produced by the highway systems template in PHP7.

@Jdforrester-WMF - That makes sense, so I guess the next step is to look at how to get the JSON and modify the template code to make it output correctly.