to sheets,
It's normal for the lockup to come off when in that driving condition.
Most drivers never catch it happening unless they are closely observing the tach.
This has been a feature since at least 1984 with the first EAOD transmissions.
It is done as an antisipation of braking to keep the motor from driving the vehichle in a panic stop or whatever.
A timer in the PCM will cause a relock after just a few seconds if there is no further braking action. This is triggered from the brake signal most of the time..

 

I've been driving it that way for 8 years. Its good to finally know that I ain't gotta fix it! Thanks for the info.