Insurance companies oftentimes issue "drafts" not "checks." They need not comply with the midnight deadline; i.e. the item may be a long time coming back to you. A Regulation CC hold for a missing endorsement that was applied after the deposit was accepted is questionable because the defect was obvious at the time the deposit was made. In any case a hold attributed to Reg CC cannot last in perpetuity.
Talk to your bank's legal counsel. He or she will probably want to reach out to the insurance company (now) and then the bank whose endorsement was required. What they want will be "fact driven," but you are going to hold on to the funds either way.
You may not feel "lucky," but it is very fortunate that someone caught this.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.