...get your old butt in here...
Ha ha....you said 'butt'!
Mr dv8 and myself have both been perusing over this problem since the moment we both decided to give up.
I tried several different ways in an attempt to find the root of the problem, paths, fills, strokes, even a large brandy and cigar but wothing norked.
dv8 suggested that there was a big difference between using Edit > Stroke and Style fx.
I'll be the first to admit I hadn't noticed this before.
Edit > Stroke does, for me, give a much better result, not dissimilar to your 'correct' example.
Using Styles does, for me, replicate your 'incorrect' example.
Along the way to solve this I've also tried examples using "Inside" when stroking, which gives a better edge but a rough inner.
Using "center" also gives different results.....as does "Outside".
I've also tried "Outer Glow" and even "Drop Shadow" as alternatives to using stroke....each with differing results.
From the tests done and the results obtained I can only deduce that using Edit > Stroke uses anti-aliasing and that Styles fx stroke does not, and, as far as I can tell, has no option to do so.
If you are looking for a specific "answer" as to why things are now different for you then you may be out of luck, anything could have happened to your system since it last worked and as nobody else seems able to replicate a 'good' version its doubtful you'll get a definitive solution.
Personally I cannot do any different than your 'bad' example....despite several Barge Landy's, so I don't know how you did it before.
MrTom.