The way I see it the Bulgarian spearmen are wearing almond shields, which are basicaly just an a bit more compact version of the kite shields. From the few Central European sources I have seen that both were replaced by the heater shield during the 13th-14th century, so the very small and strictly triangular shields.
I also looked for Balkanese sources from the 14th and 15th century depicting kite / almond shields, but I dont found any. Can you maybe post a few here?