Employer mistakenly changed my insurance plan

My husband and I just realized this yesterday. I had to go to the ER at the beginning of the year and we just got the bill. We didn’t understand why it was so high when our insurance should have covered much more of it. And the deductible amount was much higher than what we knew our deductible to be. So we looked into it and turns out we were put back on our old insurance from 2024 rather than the one we enrolled in for this year.

I had a baby at the end of last year, literally 5 days before the year ended. So in mid January we contacted my husbands employer to have them add our son onto our insurance, not only for this year but for the 5 days last year so his hospital bills and doc appointments would be covered from 2024. Well from what I could tell, we had our new insurance plan up until we made that call to my husband employer to add our son. I can tell because our new plan had an HSA and the last time it was contributed to was January, before we made the call. So somehow when they added our son they must have switched our coverage for this year.

My husband is going to reach out and try to get this figured out today. But, I’m curious what you all think. He doesn’t think they’ll cover my ER bills from the beginning of the year with the insurance we should have had because technically we had this other insurance. I think they should back date it and say it was effective as of the beginning of the year because it should have been. Wasn’t our fault that they made a mistake.

What do you think? Is it possible for the huge ER bill to be covered under the insurance we should have had? I know they’d have to rebill it, but would it be possible for the company to switch us back to the plan we are supposed to be on and have it be effective from the beginning of the year rather than from this point forward?