No Man's Sky

It's just buggy AF - I've found that if I delete decorations when in the Corvette building mode, I often don't get refunded the cost, but I do get refunded if I delete them while walking around my ship.

Similarly I accidentally restored saved modifications today and it deleted a few thousand decorations worth of Carbon and Sodium

Sounds to me like this buggy update being this buggy update

I’ve built some invalid designs, such as missing cockpits and it saved fine, although that could be because it was a corvette I had finished beforehand

did you go back to the corvette workshop in the same space station that you started it from?

I found that if I change stations while there is an incomplete design, it will not follow me to the next station. Once I finished a new design, my old parts ended up in the cache box.

I have not done much with corvettes yet, so I might have misunderstood, but did you switch your ship to the corvette and then try the builder?