Steps to replicate the issue:

Visit the 3 addresses, pay attention to the PNG map thumbnails:

1. https://zh.wikipedia.org/w/index.php?oldid=89796014&variant=zh-tw
2. https://zh.wikipedia.org/w/index.php?oldid=89796014&variant=zh-cn
3. https://zh.wikipedia.org/w/index.php?oldid=89796014&variant=zh

What happens?:

The first marker is missing in the zh-tw link. The third marker is missing in the zh-cn link. All markers are present in the non-converting (zh) link.

Explaining the situation further: The first marker title is a link using zh-hans characters. The second one is the plain text of the first. The third one is a link using zh-hant characters.

Even though the marker is missing in the thumbnail, the interactive map can display the marker normally (after clicking on the map image).

What should have happened instead?:

All markers should be present regardless of the variant being used.

The only workaround I have identified is to wrap the link with the -{text}- syntax that disables conversion for the enclosed text: In https://zh.wikipedia.org/w/index.php?oldid=90374000&variant=zh-cn, only the 3rd map thumbnail shows the marker. The 7th/last map re-enabes LanguageConverter in the displayed text of a piped link, and breaks the thumbnail again.

Software version: 1.45.0-wmf.25

Other information:

This issue can be reproduced using historical revisions, but not when previewing.

It might be related to T246314: Map cannot be displayed when Simplified Chinese characters are present.

If the latitude and longitude is not specified for the <mapframe>, the thumbnail will display the sea at 0°N 0°E when the marker is missing. This is the situation met by the original reporter on zhwiki (discussion, sandbox).