If you haven't done so already, I'd say it's time to call Quicken Support on the phone and request help. Let them do a screen share session with you to review the situation on your computer, to determine the nature of the problem and to figure out how to fix it. I have no idea what should be done to clear this up.
Quicken Support:
I have been having this problem for years, ever since moving to Quicken Windows 2014 Premium. Every time I run a validation, I get the messages regarding the same security, which has always had a share price of $1.00 and never a stock split. The validation is reporting three 'missing stock split' for the same security with dates before the account was even open and had transactions recorded. I have reviewed every transaction for this security in three accounts which have held the security and all price history and all transactions are recorded correctly. The 'missing split' is only reported for one of the three accounts.
I've communicated this to the developers for years and they will never respond or acknowledge an issue.
Adding in the 'fake' 1:1 split as an experiment did remove one of the reported errors, but we should not have to do this.
For starters, Quicken 'validation' should not assume it is correct and make arbitrary changes to our data without our review and approval.