Hi Jurkiewiecz,
Greetings from Syncfusion Support,
We have reviewed the provided details and created a sample to
validate the dependency line rendering in the Syncfusion Gantt Chart. Based on
our verification, the dependency lines are rendering as expected after
installing the Microsoft’s Fluent UI. We have attached both a sample project
and an image reference for your review:
Image reference:

To help us accurately identify the cause of the issue and address your
requirement, we kindly request you to share the following information:
- Provide the complete Gantt chart implementation code.
- Specify the Nuget package version and script version of the
Syncfusion Gantt chart in use.
- Share the details about the localization for validation.
- Share us the steps that you have followed to install
Microsoft's Fluent UI(to cross check if you have missed
out any) ? Share those details.
- Replicate the issue using the provided sample and share your
observations.
Sample Link: https://www.syncfusion.com/downloads/support/directtrac/general/ze/Localization-with-dynamic-culture
Once we receive this information, we will be better equipped to identify the
root cause of the issue and provide you with the necessary assistance.
Regards,
Ajithkumar G