Did a fresh install of Sample Charts.
https://apex.oracle.com/pls/apex/r/jaagup/sample-charts/gantt
just added a "no authentication" authentication scheme.
Go to Gantt, scroll down to Multiple Tasks per Row
No matter what dates I pick, the chart just shows nothing. Other Gantts display nicely. So what could be wrong? Some unexpected browser/NLS-settings? Tried Firefox, Edge.