Hi nramos,
I reviewed relevant files from the code you've attached. Since it is a huge and not runnable, all I was able to do is check it visually for obvious problems and try your configuration and JSON data in our example.
JSON data that I found in components-vue - copy/src/components/Gantt/examples/builder_project.json
file had incorrect format, which I had to change to use it. there is no "rows"
sub-object in the hierarchy, but I believe you know that and handled that because you have it working on your screenshots. All about data format here: https://bryntum.com/docs/gantt/#Scheduler/guides/data/crud_manager.md
Another problem with JSON data is empty objects, that causes errors in console.
After I removed them I was able to use JSON you sent and test it. I am not able to replicate the problem.
I believe this is runtime problem that we have to reproduce to know the reason and help you. It will be great if you make a simple and runnable app to replicate the problem.
Please, check the console if any errors appeared. Try to remove watchers that updates data in your Gantt, check if it happened after initial data loaded. Try to use debugger to see the data in the store, when it updated, what data was loaded to the Gantt, where from these null dates come.