@SundanceRaphael I tried purging the page and it now appears for me

same trouble with Test= {{REVISIONUSER}} for me, but using Test= {{subst:REVISIONUSER}} works totally fine.
https://de.wikipedia.org/wiki/Spezial:Beitr%C3%A4ge/Anoop_Rao

And if you try test= {{REVISIONUSER}} --> save --> purge (append ?action=purge to the end of the URL) - does it display then?

I can confirm that after purging the {{REVISIONUSER}} entry appears, but after add a new version the {{REVISIONUSER}} is empty again.

Hm, I've replicated that too

1. Create page with test= {{REVISIONUSER}}
2. Save
3. Content shows test =
4. Purge
5. Content shows test = [username]
6. Add a line break, and some words
7. Content shows test = (line break) some words
8. Purge
9. Content shows test = [username] (line break) some words

I have a feeling this is "expected" behaviour, Help:Magic words states "Due to MediaWiki and browser caching, these variables frequently show when the page was cached"

@Samtar you refer to the Date and time section of magic words. If i remember right this functionality is very special.

@Anooprao the i can confirm that {{subst:REVISIONUSER}} but i need not a at the end something like [[user:{{REVISIONUSER}} |{{REVISIONUSER}}]]

The problem is that the string becomes empty after new revision is created. For me is the "expected" behavior is like {{REVISIONID}}, "acceptable" behavior is "Show the last revision user before purge"
The interesting fact is that in the preview all is right.