Thanks for looking into that and posting the logs, it's one of those things where anecdotal evidence wouldn't be clear enough.
There's a max number of starships per planet so it wouldn't keep that up forever. But I guess it would probably keep it up for a while.
In general it makes no attempt to avoid overdraft, as otherwise the larger stuff would never get bought. But it has to pay it back before it gets more ships (before that rule was added you could get a spire blade spawner added to every guard post on an extremely small budget, etc).
In this case I probably need to make the starship budget separate again, possibly shared with guardians so it's not just a straight buff. That will probably mean it just gets to buy one each entire reinforcement cycle, but then it won't get in the way of fleet ships.