When creating a Timeline chart inside a Google Apps Script web app, the custom HTML tooltip column role is completely ignored. The chart falls back to the default tooltip.

This is happening even though the style column role in the exact same DataTable works perfectly.

Expected Behavior:
The chart should display the custom HTML string provided in the tooltip role column when a user hovers over a bar.

Actual Behavior:
The chart ignores the tooltip column and displays its default tooltip (showing only the bar label and times).

Minimal, Complete, and Verifiable Example (MCVE):
Attached is the complete index.html file for a Google Apps Script web app that demonstrates the issue. The Code.gs simply fetches data from a spreadsheet. The buildDataTableAndDraw function shows the DataTable being created with both a style and a tooltip column. When run, the colors from the style column are applied correctly, but the content of the tooltip column is ignored.

Code Example.rtf