What is the impact of a backwards connected hot water radiator?
I can't say I'm familiar with your setup, but only with big old radiators &amp; baseboards of the same principal &amp; 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 &amp; 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 &amp; in is out &amp; out is in regardless of what end &amp; what pipe is where.

Don't prejudge &amp; 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 &amp; 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 (&amp; he/she forgot about the reversed plumbing). I don't know the entire situation, I'm just explaining &amp; not excusing, a proper repair is absolutely necessary.