Both graphs use the same graph ID, which is probably causing issues? The ID is generated with $hash = sha1( FormatJson::encode( $data, false, FormatJson::ALL_OK ) ); in includes/ParserTag.php. I don't know if that is needed.

It was already the case that it used the same graph ID (that's why I had that test case on my local wiki ;) ), and yeah, that'd be my guess as well that this is what causing the issue. Identical graphs are not without their issues, see T312242 and related.
The hash on the data may not be necessary per se, but it feels like a reasonable solution to ensure ID unicity (... up to using the same data, but then the same graph would be rendered, so it "shouldn't be a problem™") and ID stability, I guess.

Change 911937 had a related patch set uploaded (by Jdlrobson; author: Jdlrobson):

[mediawiki/extensions/Graph@master] Map schema should not have side effects and map marks field

https://gerrit.wikimedia.org/r/911937

Change 911937 merged by jenkins-bot:

[mediawiki/extensions/Graph@master] Map schema should not have side effects and map marks field

https://gerrit.wikimedia.org/r/911937

Change 911802 had a related patch set uploaded (by Jdlrobson; author: Jdlrobson):

[mediawiki/extensions/Graph@wmf/1.41.0-wmf.6] Map schema should not have side effects and map marks field

https://gerrit.wikimedia.org/r/911802

Change 911802 abandoned by Jdlrobson:

[mediawiki/extensions/Graph@wmf/1.41.0-wmf.6] Map schema should not have side effects and map marks field

Reason:

Too many patches would need to be backported for wmf6 so I am shifting focus to wmf7

https://gerrit.wikimedia.org/r/911802