This CSL snippet (angle brackets removed so this comment displays)

group delimiter=" "
text term="retrieved"
text term="from"/>
text variable="DOI"
/group

yields "retrieved from 10.1038/s41398-020-01089-6"

However this CSL snippet

group delimiter=" "
text term="retrieved"
text term="from"
text variable="URL"
/group

yields nothing

The Zotero record has both a DOI and a URL
DOI: 10.1038/s41398-020-01089-6
URL: https://www.nature.com/articles/s41398-020-01089-6

I've tried a lot of variations - any variable I use except URL fills in correctly (e.g., volume)

Is there something specific about accessing the URL variable I should know?

Is this issue related?
https://github.com/Vitaliy-1/JATSParserPlugin/issues/56

As context (if helpful), I'm modifying the access macro of the AMA 11.0 CSL.