Insurance is denying a claim stating we don’t have coverage because they got the bill after we switched to a new insurance company

My husband and I have been fighting an old insurance company for almost 8 months regarding a claim for my son.

Background: My husband’s company switched from Insurance A to Insurance B as of April 1st this year.

On March 29th, we took my son into the emergency room. Shortly thereafter we got a call from the hospital saying that our insurance denied the claim. Insurance A says that they are not responsible for bill because they received it after the insurance switched over and Insurance B is telling us that because they didn’t have any coverage until April 1st that they are not responsible for the bill. I completely understand that Insurance B would not be responsible for this giving the circumstances but what I don’t understand is why Insurance A wouldn’t be responsible for the bill seeing as we were covered by them on the date of service.

Luckily it’s not a large bill and we could pay out of pocket for it however I’m extremely frustrated that the insurance we supposedly had is telling us we have no coverage. Can they do this? Can they deny a claim with a date of service inside of a time we were covered because they got the bill after our coverage with them expired?

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