In retrospect, it would not have been a solution in this particular case, but in case 2 very similar objects are behaving differently, we usually start with a comparison of the stored parameters of the instances.
You can check the current parameter set of a placed object by using the “Dump Library Parts of selected Element” command, from the Library Developer menu.
This command writes all available parameter and global variable data of the selected object into the Report window. You can copy-paste it from there to any file comparison applications to look for differences in the 2 sets. Make sure to remove all previous notes from the Report window before using the command for easier difference-spotting. Works on one selected object at a time.
In case you already knew this, sorry for the long post. Hope it helps next time.
Next step in a case like this is the Compatibility options check. Bingo. Well done, Barry:)