@jwang identified an issue while working on the QA WebUIScroll porting to the new metrics platform (T352342). The schema for mediawiki_web_ui_scroll_migrated is currently empty, which is unexpected. Although the table was created successfully, there seems to be a failure between the creation of the table, the validation of the events (there would be Logstash errors if validation failed) and what happens after.
Upon further investigation, we also observed that the mediawiki_web_ui_actions schema does not have saved events (T353029).
Update: On checking the Network tab in prod, we're also not seeing the Metrics Platform events.