Always replace in pair.
In some countries TUV-MoT does not approve different brand/model/season tyres on same axle. But different sizes on same axle is prohibited in all across EU. So there is not many other options than to change them both.
No such rule here in the UK.
I do recall many years ago that having crossply and radial on the same axle wasnt allowed, but that was many years ago.
I can understand to replace in pairs if both have worn evenly, but to change both if only one has developed a fault and the opposite one is only a few months old with little ware seems a bit OTT.