Are you talking about this LD release that had a Dolby Digital audio track?

If it really was remastered from the 70mm audio, it's quite possible that the surround (rear) channel is mono - this was the configuration for 70mm 6-track sound around that time (left, centre, right, surround, LFE left, LFE right).

Dolby Digital tracks can only have one LFE channel, so the audio would be actually be 4.1 (or maybe 5.1 with "dual mono" surrounds).

Anyway, I don't have the laserdisc, nor the facility to capture Dolby Digital streams off LD. But this sounds like an interesting bit of history to explore.