Hi Tomasz,
Thanks for using Syncfusion Support.
We have analyzed your code snippet and attached image, but we could not able to replicate the issue in our side. By checking the provided image we suspect that you are doing client side chart customization in the Gantt. Can you please share us your client side code to provide you with the proper solution. We have also prepared a sample for your reference with the given scenario.
Please get back to us if you need any further assistance.
Regards,
Jesus Arockia Sankaran S