I know what you mean. It was always kind of sad that when Kay would flashback to the past after JW took over, they could never show some of her best scenes with Jill, since BD was no longer playing the part. JC in her book said BD could be a handful at times, she still loved working with her and people behind the scenes did her wrong.
She said she had to play that up because she no longer felt Jill was a heroine, or something along those lines. Whatever happened it seemed like things started spinning out of control somewhere between the early months or first year of her return and 1986-1987. It's a shame, since as much I do love Jess as Jill, I think Brenda at her best really got the heart and emotions of Jill (as shown in some of those '80s flashback episodes and more raw moments).
I always wondered what went on with BD and the show in her second run. What bits I saw of her 70's run, she was more human. In her 80's run (when I was watching then) she came across as a campy caricature and cheap copy of Alexis on Dynasty. At one point she started speaking with kind of a eurotrash accent.