Well, technically, you CAN insert any enemy anywhere you want, but it doesn't mean the correct graphics will be loaded for that particular level. Anyway, it's also a guess from me, because I'm not familiar with arcade graphics. Yet, it's the most likely explanation, as far as I know...
The PPU is the same for the 3 first groups of 16 levels, i.e., levels 1 to 48, and it doesn't have the Drunk graphics, although the Invader graphics are there. Group 49-64 has Drunk graphics, but you lose Banebou and Pulpul graphics. Group 65-80 goes back to the former set, so no Drunk. Group 81-96 has Drunk, but no Mighta and no Banebou. Group 97-112 has Drunk, Banebou and Pulpul, but no Mighta and no Invader...