I've been reading through fighter martial archetypes, and I am a little confused. As far as I can tell the Battle Master Martial Archetype is missing its level 18 feature, unless they're counting the increase in the superiority dice size. Is it actually missing, am I missing something in the PHB, or is it just the superiority dice size?
No, Battle Master isn't missing its level 18 feature.
You found it: increasing the superiority die size is what a Battle Master gets at level 18.
You seem underwhelmed. I'd point out that these high-level features strike me pretty variably: Beast Spells (druid) hits me as awesome; Indomitable Might (Barbarian), not so much.
Summed across levels 14-19 the various classes strike me as well-balanced, but in any class sometimes the features strike me as progressing unevenly. But your mileage may vary.