setUserVariant has two modes of working. Roughly, its structure is:

if ( mw.config.get( 'wgGEUseMetricsPlatformExtension' ) ) {
    // Use mw.xLab.overrideExperimentGroup to override the variant for the user
} else {
    // Use the `growthexperiments-homepage-variant` user property to override the variant
}

wgGEUseMetricsPlatformExtension should be set to Util::useMetricsPlatform(). Unfortunately, we do this in SpecialHomepage::execute, which means the wgGEUseMetricsPlatformExtension variable is only populated on Special:Homepage. On all other pages, it returns the fallback value. The default fallback value is null. In JavaScript, null casts to false, which means the user-property-based branch of the code runs. That branch successfully changes the user property, but...we do not actually use it for determining the variant, so the user sees no change.

Change #1204843 had a related patch set uploaded (by Sergio Gimeno; author: Sergio Gimeno):

[mediawiki/extensions/GrowthExperiments@master] fix(ge.utils.setUserVariant): return wgGEUseMetricsPlatformExtension JS var

https://gerrit.wikimedia.org/r/1204843

Change #1204843 merged by jenkins-bot:

[mediawiki/extensions/GrowthExperiments@master] fix(ge.utils.setUserVariant): add GEUseMetricsPlatformExtension JS var

https://gerrit.wikimedia.org/r/1204843

@Urbanecm_WMF tell me if I'm looking into this correctly.

After confirming that I'm in the revise tone control group, I now notice that the ge.utils.setUserVariant() can be executed on the Main Page in arwiki beta.

Executing ge.utils.setUserVariant('growthexperiments-revise-tone_control') reloads the page as expected.

Nothing errors out.