FDF Research base upgrade?

[Griede Starless]

SO The FDF research base is listed as a teir 5 military base.
if i am understading this correctly, it should only upgrade to Teir 6.
However, once FDF Brigade is unlocked (listed as Teir 4) this will pop up as an "upgrade" to the research base. is there a typo or bug here somewhere?

[Gilmoy]

I don't think it's that strict.
Upgrades from different trees aren't necessarily linear.

FDF Research Base is in the Superscale Construction tree.
FDF Brigade is in the Stellar Warfare tree.

FDF Brigade does defend +21 levels, house +2 pop, and generate +1 EP,
compared to FDF Research Base. That does suggest that it's actually the higher tier.