Answer
This has been confirmed as a genuine defect in the Lobby Element Designer's multi-series Line Chart handling (the "Split on" column behavior specifically), fixed starting around the 24R1 release — check whether your current release already includes the fix before spending more time adjusting the chart's data-source or split-column configuration, since the same setup that worked in the older on-premise product isn't expected to render correctly on an affected pre-fix Cloud release.