Since 5.5 the FBX import now uses Interchange Framework, so unless you are forcing legacy fbx importer it is Interchange that is managing your FBX imports.
In that context you should go into Project Settings > Interchange > Import Content and tick the “Show Reimport Dialog” option.
Thanks, that one worked. Bit weird that this setting still exists under General > Miscellaneous > Import. For reference, this is a fresh project created under 5.6. No plugins added or removed yet.