Hello,

your resource is possibly planned for other activities, therefore it is only available on its dates.

Hello,
All resource allocations are being handled by Projeqtor, therefore if there are already tasks during the same period of a resource, they may be trimmed by the Projeqtor and the new ones are replaced into the trimmed areas. In reality, no one repeatedly works just an hour in a week for an activity of a project.
Can it be a constraint of Projeqtor's algorithm that attempts to make the activity finish according to "as soon as possible" rule?

Thanks in advance.

Yes, it is possible that this is the "as soon as possible" planning mode causing the resource to be planned like this. There is the allocation of the resource to the project, the capacity  and calendar  of the resource which can contribute to this result.

Is seems your resource is allocated to project with a very low allocation rate.
Try and set it to 100% to see changes.