if you have a tech requirement that depends on the level of an upgrade, the requirement is ignored, as long as the upgrade is researched once. You can see this bug in effect in maps such as Barrens TD (see on hive workshop, cant post links for some reason)
To give an example:
Iron Forged Swords (Level 1)
Steel Forged Swords (Level 2)
Mithril Forged Swords (Level 3)
if you have a requirement with 2 levels
Improved Health (Level 1) - Requires Iron Forged Swords)
Improved Health (Level 2) - Mithril Forged Swords
The second upgrade (Improved Health (Level 2) - Mithril Forged Swords) wont work, the player can get the upgrade straight away.
I hope that makes sense
Cheers.