Indeed very normal and lots of threads discussing this phenomenon, which has been going on for many years. Essentially, early on after flights get released, BA tends to only release seats in relatively to very expensive travel buckets. Any release of availability in cheaper buckets comes later. So really, unless we are talking about award seats or unless you really need to make sure you have a seat on a very specific flight and want to book it at the earliest opportunity, it is usually not a good idea to book within a month or two of the 330 day release.