This is a consequence of Build Priority having two purposes. Everything under construction has a build priority that overrides the priority of (default priority) constructors that are building it. Every constructor also has a build priority that determines its own expenditure. Constructors (including Caretakers) switch from having the former type of priority to the latter when they are built. The default constructor priority can be set in "Settings/Unit Behaviour/Default States/Economy" or by space clicking a Caretaker and clicking "Edit Behaviour". Inheriting the nanoframe priority sounds like a fine idea too, so I made a ticket https://github.com/ZeroK-RTS/Zero-K/issues/4782