Standard
The -145 always turns in early unless the waypoint is marked up as a 'flyover'. POMAX is not labelled as a F/O in the box so unless a pilot actually marks it as a f/o on the flight plan it will turn in early.
The reason for this is simple. the a/c follows the track line into a waypoint correctly. It wishes to leave the waypoint on the correct track line also. As it can't turn on a point, it leads the waypoint turn to avoid 'going through' the outgoing track line. More degrees of turn= earlier commencement of turn. 3 or 4 miles is entirely typical for any en route turn.
Suggestion. During the work on 'G' why not shut Foggy Bottom and let us all fly out of Bristols proper airport?