As per the NS manual, and common knowledge, recycling is supposed to give you back half of the cost of the structure, but in the case of upgraded structures this is not the case. You are awarded half of the cost of the upgrade, not the full structure cost. Is this a bug, or an intended aspect?

There's basically only 2 structures that can be upgraded: armory and tf.

Armory costs 10 res, the upgrade to Advanced Armory costs 30 res, so total structure cost is 40 res, but recycling only gives you 15 res (half of the upgrade cost).

Turret Factory costs 10 res, the upgrade to Seige Factory costs 15 res, so total structure cost is 25 res, but recycling only gives you 7.5 res (half the upgrade cost).

I don't know of any commander who would recycle an Advanced Armory, but Seige Factories get recycled regularly and you're missing out on 5 res, IF this is unintended behavior of course.

Related, perhaps, is that electrification of RTs and Turret Factories does not have any effect on the res awarded upon recycling.