OK, I am now sure it is an issue with the model itself, not a problem with the aircraft data.ini.
I deliberately deactivated the rails intended for Python 3 and then took a screenshot of the rails for Sidewinder.
I am sure the aicraft data.ini is calling the correct rails, but the inner and outer IRM rail under the left wing appear to be closer to eachother than those under the right wing.
The effect is missiles appear as if improperly attached (floating). See attached screenshot.
Using the supports under the inner pylon as a reference point, you can see the rails under the right wing are separated and do not touch the supports, while the rails under the left wing clearly touch them. Apparently the rails under the left wing need to be a little bit more separated from the pylon to be identical to the rails under the right wing.