I wonder that:

The convergence with the message "reversed flow in 6 faces on outflow" is a valid convergence or not?

In other words, when it occurs, shoud we accept it or try to re-run to overcome it?

Reverse flow is problem dependent. if you have your outlet at such a position that there are vortex in your flow physics, you will surely get reverse flow in your simulation. In other words, consider flow over a cylinder and now you ahve your outlet in the wake zone of the cylinder, then you will get reverse flow. You can increase your outlet location, though this is conditional. If you coudl tell your geometry, the situation would be more clear to comment.