When building a hot water heating system the plumber has connected one of the hot water radiators backwards, i.e., the loop comes in at the bottom of the radiator and leaves the radiator on the other side at the top. Unfortunately, this has been discovered only weeks after the installation and it's not easy to fix (there is no access to the pipes making up the loop anymore). What is the impact of such a backward installation? Does it affect the heating, efficiency or maintenance?
I can't say I'm familiar with your setup, but only with big old radiators & baseboards of the same principal & setup. Your problem is no problem in those systems, they are just to circulate water in one direction of a daisy-chain. The only definition of inlet was which side a flow management valve was installed on.
But, definitely get the plumber back out to inspect his work with you & explain how the system works. In your case I would assume it also doesn't matter at all because your system is actually zoned (it better be), meaning that this radiator might be its own loop & in is out & out is in regardless of what end & what pipe is where.
Don't prejudge & stay calm, just ask him back out to briefly explain the system. You'll need him friendly when the system goes active to adjust the zones for your maximum comfort & overall house balance.
Due to your response indicating this is indeed the start of a zone, the best fix would be to just swap the pipes at the manifold in the basement (or wherever) as the other radiators after the incorrectly installed unit are just flow-through units. Proven by their working perfectly with the first unit being essentially bypassed almost entirely.
No cause for panic nor messy permanent repairs, like replacing the entire zone. The plumber may have initially intended the incorrect unit to be a flow-through, but had to change his plan if a zone was added after the fact (& he/she forgot about the reversed plumbing). I don't know the entire situation, I'm just explaining & not excusing, a proper repair is absolutely necessary.
I do not know if the piping has been corrected however let me make a few comments: you cannot daisy chain cast iron radiators and expect them to work/heat properly. Each radiator must have its own supply and return piping. Also you cannot series connect radiators. As to the radiator being piped backwards it will only affect the operation a small amount and probably not enough to notice. I read other answers that mention flow through and osmosis which are terms I am not familiar with for this type system. Each radiator should have been sized for the heat loss of the room it is installed in, meaning that if everything was done properly there is almost no need for controls on each radiator. Good luck