"Hide the account in transaction entry lists" box is checked and greyed out
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Even though the account wasn't closed, the op's experience does not reflect anything wrong with Quicken: Quicken is working as designed.
The option to "Hide in transaction entry lists" was checked and grayed out because the user is not allowed to enter any transactions in an online-enabled loan account.
And the original loan account couldn't be re-activated because Quicken cannot "re-activate" any loan account. Quicken cannot "Link" downloaded transactions to an existing loan account, Quicken can only create a NEW loan account. Once an online-enabled loan account has been de-activated, it can never receive downloaded transactions again.
The op says, "Have created numerous mortgage accounts prior without any issue ....".
But apparently none of those mortgage accounts were enabled for downloading; if they had been, they would all have acted just like this one did. So there was no "issue" in this case either.