B737-800 flight attendant reported the main cabin door handle would unseat during cruise flight.

Narrative

Before departure; the 1L door handle was confirmed to be in the proper; fully flushed position during our ground checks. However; shortly after takeoff; we heard a loud sound of pressurized air; and I noticed that the door handle began to lift and prop upward. I had to physically hold the handle down to keep it in place during the flight.I immediately notified my colleagues; and we all inspected the door together. The pilots who brought the aircraft in also confirmed that this was not normal and mentioned that they had already written it up. They also informed us that the aircraft's system was not giving any notification that the door was open; even though the handle was not sitting flush -- which raised additional concern about the accuracy of the door sensors.Upon landing; I rechecked the door; and the handle was once again fully flushed--indicating that the issue only occurs in flight. When maintenance came on board; we explained exactly what was happening. The mechanic responded; Yeah; no; that doesn't sound good." However; he also stated that his instructions were only to open and close the door on the ground to confirm it was safe; and that was deemed sufficient to clear the aircraft for the next flight. After the aircraft was cleared and departed again on the next flight to ZZZ; the same issue with the 1L door handle occurred; confirming that the problem was not resolved."