I've just upgraded my previous Lightroom 4.4.1 catalogues to Lightroom 6. All seems well, apart from any images that HAD redeye reduction applied to them in v4.4.1. The darken amount is not being applied to any of them.
Thanks for the reply. If its a long standing bug then its surprising to me that its still not been fixed, as its obviously then a bit of an oversight. In the end I actually used a plug in (in Lightroom 4) to list all of the images in my catalogue that had red eye reduction and have redone them all in Lightroom 6. Its a workaround but not ideal (not sure if plugin works in Lightroom 6).
Luckily I'm just a 'own family' photographer and only have sub 400 photos in total in just 1 catalogue, and so not that hard to do (with only my early jpegs affected, lol).
Plugin (if any body else wants to use the same workaround): Jeffrey’s “Data Explorer”