I have to say that this has happened to us recently on LGW flights too... seems like it happens more on those than on LHR flights, but that may just be coincidence.

On two occasions, my partner (gold status) assigned us both to exit row aisles, same row, opposite sides. When we went to check in, his seat was intact, I (no status) was moved to 26B or something ridiculous.

In each case I was able to at least get moved back to a middle seat on the exit row, but geesh... something so simple that can really make your good customers feel like they have a bit of control of their in-flight experience, and we can't even get that right. *sigh*