Overall, the total block count is correct, but the given value includes the perimeter made of glass, whereas it should actually be made of casings.

modloader: neoforge
mod ver: 1.21.1-7.7
mekanism ver: Mekanism-1.12.1-10.7.14.79

like this with glass is ok

Please more detail.
I can't found any wrong

9x9x14 reactor

spec:

create copied schema

schematics.zip

calc for reactor casing block normally (with glass check):

9*9 = 81 (bottom)
13*4 = 52 (edges + 4 blocks in top)
7*4 = 28 (top)

81+52+28 = 161

total glass for a 9x9x14 fission reactor:

7*7 = 49 (top )
7*12*4 = 336 (other faces)
49+336 = 385 (total glass wihout extra block)
385 -6 - 4 = 375 ( total glass block)

ps: as i remember, boiler has the same bug

Reactor glass can be used at bottom, Structure glass is too.
I tested to build 9x9x14 fission reactor in manually.

I found the issue

it was a misunderstanding on my part. The floor is also made of glass when that option is checked (hence the difference in block count).

I’m closing the ticket. Have a great day ^^