Why are certain billable tasks missing from task proposals when entering time?

In “Configuration > Activities” and “Configuration > Billable task codes”, activities and billable tasks can be associated with each other, to limit entry to certain tasks based on the activity you are doing for that affair. You can [Ctrl] + click to associate multiple activities and tasks together.

Tempolia screenshot: The billable task codes that control suggestions during time entry. — The billable task codes that control suggestions during time entry.

Either you do not link any activity to any task, and all tasks are selectable when entering time.

Tempolia screenshot: Task selection in time entry. — Task selection in time entry.

Either you link them together, and then your configuration must be complete, each activity being linked to N tasks. And when a collaborator enters time, he chooses a case which is associated with a given activity, and this activity then limits the list of selectable tasks.