You can look up information on the following site for activity on on 14 Sept 1943.
http://www.hellothomas.org/eighthaf/...ns_by_date.php
From that you can see the following.
VIII Air Support Command Mission 57: 108 B-26s are dispatched to the Woenisdrecht Airfield in the Netherlands and Lille/Nord Airfield in France; they are recalled due to weather; 18 B-26s are damaged; casualties are 3 WIA.
Scroll down on the page and you can see what groups flew the mission:
the 386th, 387th and 322nd. Also no "losses" are recorded.
If the mission was scrubbed, it may be that it was not considered a "combat mission" for all the participating groups. Note that you can click on each of the groups in turn and get all their missions listed and also all non-combat losses for the war. While each of the groups shows non-combat losses for the war, none of them show a loss on that date. Since the description given in the 386th log seems so "definitive", it may have happened.... but the records just don't seem to support it.
Note also that a P-47 was lost that day (following down in the fighter group activities). That turns out to be 42-22531, which was lost at sea. Lt William H Alm was KIA. He is still listed MIA. Based on some times given in the MACR, Alm's loss seems to have happened after the the ditching recorded by the 386th, so it does not appear that they somehow witnessed him going in. So..... still a mystery.