It's been happening since covid I believe. Factions that gain a lot of demand because of new rules (but not new ranges). I think this because they tend to sell out due to likely just not selling well in the past, but that is a guess on my part.
I think new ranges, however, since they are built up for a big launch release - don't sell out that easily. Some kits still do - but I believe they return much more frequently in availability.
Could also be brexit.