Hello!
This is probably not a bug. The massive conquest gave you significant amounts of Aggressive Expansion (AE), which reduced the happiness of your pops - particularl foreign culture ones - significantly. Given that only a tiny fraction of your pops are Greek / Macedonian, this means that the output of your Freemen dropped significantly, reducing your maximum manpower, scaling your current one down to the new max.
This is probably not a bug. The massive conquest gave you significant amounts of Aggressive Expansion (AE), which reduced the happiness of your pops - particularl foreign culture ones - significantly. Given that only a tiny fraction of your pops are Greek / Macedonian, this means that the output of your Freemen dropped significantly, reducing your maximum manpower, scaling your current one down to the new max.