If I do a full model fit that extends beyond 1980 to present, it appears that a perfect phase reversal occurs from 1979 to 1996, a span of 17 years -- the purple bar in the chart below. A phase reversal in that a positive forcing has the opposite effect on the sign of the result.
I also added a level shift to the phase reversal. This mod is an experimental hack in lieu of trying to understand what is going on. The time series again reverses phase after 1996, with the QBO back in sync.