gantt charts in confluence drawio: "happens" uses wrong date

Describe the bug
When using gantt diagrams within Confluence Cloud with drawio, the syntax is not interpreted correctly for the "happen" keyword.
Check out plantuml docu how the date should be handled: https://plantuml.com/gantt-diagram#outa0ef7ea1711c2a6a1d55c31efc0e36bb
Defining the date shall place the milestone there. On drawio integration, it is normally placed one day to late:

• [fix] happens 2026-10-03 will show the "fix"-milestone on the 4th.

To Reproduce
Steps to reproduce the behavior:

1. Insert drawio macro
2. Click Arrange > Insert > Advanced > PlantUML...
3. use this short example:

@startgantt
saturday are closed
sunday are closed

Project starts 2026-10-01
2026-10-13 is closed

[fix]     happens 2026-10-03 and is colored in SlateGray
[frame]   requires 10 days
[frame]   starts at [fix]'s end and is colored in LightGray

@endgantt

4. The fix-milestone is on the 4th:

Expected behavior
The milestone should be on the 3rd.

draw.io version (In the Help->About menu of the draw.io editor):

• draw.io version 31.4.5

Desktop (please complete the following information):

• OS: current Windows 11
• Browser: current Chrome
• Browser Version: 153

I tested the problem in incognito/private mode with all browser extensions switched off, write "yes" below:
yes

Additional context

Start by reproducing the supplied Gantt example through Arrange > Insert > Advanced > PlantUML in the Confluence Cloud integration, then trace the handling of the happens keyword and its date. Done means the [fix] milestone defined with happens 2026-10-03 is rendered on October 3 rather than October 4.