It’s intended behaviour, you haven’t obtained the building until it’s assembled. The buildings age is always determined upon obtaining the building
If this wasn’t the case you’d have to seperate each individual fragment by age as well
If buildings kept in the inventory are differentiated by age, the fragments also needs to be separated by the age.
Logically, IMO collecting fragments from space age mars, keeping in the inventory for the players wish to move till space age Venus and assembling the fragments will yield the building with space age Venus irrespective of fragments when they collected is not to be intended behaviour rather missed use-case by Dev's regarding fragments.
Official wiki for fragments last updated very long back, and didn't cover latest changes. so anyone can argue like current behaviour is the acceptable one, since no detail mention in the wiki's regarding fragments age.
Fragments shouldn't make the age conversion like this. Fragments behave like bank deposits, once you deposited in the inventory, it can be used at any age for assemble.
Why they allow fragments assembling to work like this when stores building kit stores building to inventory with same age and player move to next age, but building if placed on the new age it still points to old age. Why did they (dev's) forgot to implement that logic or intended behaviour to point on the current age?
It's a mistake according to me, when assembling the fragments points to latest age irrespective of fragments collected age.