I didn't discover this, which I believe is an error, it was my brother in law that brought it to my attention, since I rarely do MM builds in mids.
It appears that every Mastermind except Demons does not properly display the endurance cost reduction from reduce endurance costs enhancements (whether single, dual, training, or invention origins(even sets enhancments)) for the tier 1, 2 and 3 summons for MM's. Just wanted to bring this to your attention.
@Sir Giles
Hm... It turns out they're specifically listed to exclude Endurance Cost in the Mids DB, and I'm not sure why. I think in the past Endurance Reduction wasn't supposed to work on summons and only on the powers of the pets themselves, but this isn't the case anymore. The next version of Mids will have End Reduc work on the summons.