New user, "lost" tasks

I invited some users today, and one piece of feedback: one of them made some tasks, but the tasks “disappeared” and he wondered what had happened. What happened was, the board he made the tasks on had a filter, so making them there made them disappear immediately, because they were filtered out.

The problem from my perspective is, the fact that the board had a filter, was not immediately apparent. It should somehow be clear that the board has a filter, visually.