I cannot reproduce this behavior.
Are you sure the selected resource has assignment on the selected projet ?

If you select all projets before opening Resource planning, the select resource, is display OK ?
 

yes, the view is fine when you open it the first time. It's just when you apply a filter it will say there is no data, but when you force it to refresh the view for instance by reselecting a project (even tho it has been selected the whole time) is refreshes it with the filter correctly

I cannot reproduce.
What I get just after selecting a resource is correct
 

What version are you using ?
Can you try and set use cas on Demo ?

11.3.2. But it happens pretty much in any version. I tried it with users having less projects and team members visible, and it works. So it's possibly something with users having lots of projects visible. It just returns No Data from the jsonResourcePlanning.php endpoint and there is nothing interesting logged, so i'm not sure how i can debug it further.

Please try and reproduce on demo so that we can share a use case.

I think it might be when assigned(d) is 0. It is not shown for that resource, even if it is assigned. Is this how it is supposed to be (Maybe it just worked accidentally in < 11.4 when using the initial view?) ? Is there a way to show it anyway?

indeed if the assigned load is 0, it is because this resource is not planned on the project activities and seeing a non-existent planning on a planning tool will be complicated...
It must be remembered that the assignment on the project gives a set of raw rights on the project, the assignment on the activity is to distribute work. No work no display.

Is there a way to show it anyway?

What exactly do you want to see if not the load? the list of resources on the project?

yes, we don't assign hours to each task. We just want to see every resource assigned for the task

In this case, the Assignment screen should meet your needs. Even resources without workloads are displayed there.