Sorry for reviving a slightly old thread, but I have exactly the same problem (AC12). I've had it on 2 different pc's so I don't think it's computer related.
It has always annoyed me no end. Set the line settings with the eyedropper and as soon as you let go of the alt key, the settings revert back to a random rarely used layer. The second time, everything works fine.
Also as a possible extension to this (or perhaps a totally different matter), when I set the properties of a polyline from a favourite and then switch to a line, arc etc. it reverts to the previous settings, even though the uniform settings box is checked. I can repeatedly switch between the tools and it switches between the two sets of settings, one set for the polyline and one for all the other line tools. If I set the polyline settings manually the settings are applied to all line tools properly.
Anyone have any ideas on either of these problems?
Thanks,
Matt
AC24 (7000 AUS FULL)| Windows 10 Pro | Intel Core i7-12700 @ 2.1GHz | 32GB RAM | NVidia T1000