Since I carry the center 500lb also whan hanger busting I thought that it may be related to the diffence in your loadout and mine. But it worked correctly either way on my end.
Suggest looking in game folder for settings. The file in Planes is for loadout and the file outside of Planes is salvo/delay (or at least it is on my PC). Check properties of p47d40 to see if it is read only. If you had salvo set to 10 in the past, it may be stuck that way until you uncheck read only.
Had a rampent problem with this when I migrated game from another computer.
Another possibility (very remote) is that your game is reading more than one keypress as fire secondary or reading a single key press as two.