Thanks to @Krenair we avoided having no event logging during an A/B test.
We confusingly have a global config variable called wgSchemaPopupsSamplingRate that is passed to a mediawiki config variable called
'wgPopupsSchemaPopupsSamplingRate'
This can lead to confusion like seen today in https://gerrit.wikimedia.org/r/310483
We should probably rename the global config variable to wgPopupsSchemaSamplingRate and rename the mw.config value.