This sounds extremely strange!
Just to add a data point: In the past couple of weeks, I flew over 10 segments on BA with someone who is OW Ruby (AY+ silver), and this OWR (on their separate booking) was able to reserve any seat in both Euro Traveller and UK Domestic, a week in advance already, using the BA android app. As OWS, I could do the same. We found it particularly convenient, surprising even, that this was possible even for Ruby, and got the exit row on every single leg.
It seems very odd that a BA equivalent of Ruby flying BA had no access to such a function. Could this be LH-specific?