Let's start at the beginning: Given that the balance should always be zero, what transactions, if any, should appear on the investment side?
If you mean "Cash Management Account", which would be any cash in your taxable brokerage account, WHY should it always be zero?
And, if you used the Q feature "Show Cash in a checking account", then any cash would appear in the pseudo-checking account that was created when you chose that option … and NO cash, or transactions representing cash, should appear in the brokerage account. They automatically get moved, by Q, into the checking account when downloaded.
SO, you're going to need to do a WHOLE LOT more explaining about your issue.
BTW, a Fidelity CMA is my primary checking account. And another is the primary checking account for my wife.
I have the same linked accounts as you explained, one in investment (with FZDXX as cash) and the linked account in banking which acts as my pseudo checking account. The reason I asked my original question is that I repeatedly get negative SoldX transactions in the investment side and a corresponding overage in the checking account. I believe these transactions are related to internal movement of Fidelity funds. In the end I am left to delete the negative transactions in the investment side which also removes the overage on the banking side. This has been going on for months and is completely unnecessary. There must be a fix.
There's an issue that's specific to FZDXX, use the search box at the top of the forum to find those discussions.
Quicken insists that it's a Fidelity problem. Quicken currently has no fix. Told to contact Fidelity.