Pure conjecture: The "security solution" probably wanted to ban the user for a reason unrelated to the UA string, and was only able to (i.e., the user was only

Pure conjecture: The "security solution" probably wanted to ban the user for a reason unrelated to the UA string, and was only able to (i.e., the user was only identifiable uniquely enough) because of the odd UA string. Switching to the standard UA string places the user into a state sufficiently non-unique as to be unidentifiable and thus unblockable.