So, doing curl -XPOST -H 'Content-Length: 0' 'https://commons.wikimedia.org/w/api.php?action=purge&titles=Template:Potd%2F2020-12-14_(en)&format=json&forcelinkupdate=true&forcerecursivelinkupdate=true' fixed the problem, now the original query returns the image. So, I guess this was an instance of a lost RefreshLinks job.

A followup question - why are you querying the English POTD description page instead of POTD page itself?

@Pchelolo Sounds like there was more cache invalidation we needed to do, thanks.

As for why English - I looked into it. I was hoping it was because the device language was set to English, but nope - looks like we're pulling this info as English in all cases.

To confirm: We can omit the language for this query and receive the same data, right? So the API returns identical data for this query[1] and this query [2]? If so, we can update our code to remove the reference to English.

[1] https://commons.wikimedia.org/w/api.php?action=query&format=json&generator=images&iiextmetadatafilter=License|LicenseUrl|LicenseShortName|ImageDescription|Artist&iiextmetadatalanguage=en&iiextmetadatamultilang=1&iiprop=url|extmetadata|dimensions&iiurlwidth=640&prop=imageinfo&rawcontinue&titles=Template:Potd%2F2020-12-14_(en)
[2] https://commons.wikimedia.org/w/api.php?action=query&format=json&generator=images&iiextmetadatafilter=License|LicenseUrl|LicenseShortName|ImageDescription|Artist&iiextmetadatalanguage=en&iiextmetadatamultilang=1&iiprop=url|extmetadata|dimensions&iiurlwidth=640&prop=imageinfo&rawcontinue&titles=Template:Potd%2F2020-12-14

I think it should return identical data for both of them, at least if I correctly understand how these templates are setup.

PR is up: https://github.com/wikimedia/wikipedia-ios/pull/3807

From my conversation with PPchelko regarding the task:
For now it will be enough to handle empty response (as mentioned in the desc, there is a separate ticket to improve the iOS) as it happens..., and as we've got deferred updates of the data, mentioned here - the task can not be resolved by itself except the change: "remove the reference to English." (will not eliminate the root problem, but this change is good to have from my point of view).
So there is an architectural issues wich is good to take a look in the future.

What do you, guys, think we could do else to make things better ? Thanks!

Appears to be working for me on 6.8.0 (1794)