Hello,
It may or may not be related to Gantt configuration, but it is hard to suggest what might be wrong as I don’t see your code.
Please, add your configuration in the following snippet and make sure that the issue is reproduced there:
https://snippet.dhtmlx.com/38ee1b370
Then, click on the Share
button and send me the link.
Or send me a ready demo with all the necessary Javascript and CSS files so that I can reproduce the issue locally.