I have not experienced this.
My partner and I will often, on the same PNR, take 62A and 64K (for example) on a night flight. We've never been shifted.
(As an aside, we also have different surnames, and so unless anyone looked closely at the PNR they wouldn't realise that we were on the same booking. CC will often be quite surprised in the morning when we greet each other with familiarity. Once I was told, "gosh you move fast!"...)