I must be doing something wrong, because for me the suggested "put a period '.' in the Memo text of a Memorized Payee List entry" works well. Quicken does not populate the Memo field with downloaded data when the Memo field already has data in it.
My memorized transaction list is empty. I have a list in my downloaded transactions. Consider just my Amazon purchases. Once, 2-1/2 years ago, I added a memo as to what I bought on THAT DAY from Amazon. Now, all 20 transactions from Amazon think I'm buying what I bought 2-1/2 years ago. How do I make Quicken leave the field blank instead of matching it to some arbitrary prior transaction?
And yes, since I bought that one item, I have bought other items from Amazon and put something in the memo field. Why it is stuck on this one, I don't know. But I've dozens of transactions every month where I have to fix Quicken's auto-population of Category and Memo, and I want to turn it off. Just stop filling in those fields!
If there are no Amazon entries in your Memorized Payee List, Quicken might pick register transactions … and it probably picks the oldest one …
Since that is not what you want, I recommend you memorize a recent transaction. Edit the resulting MPL entry to put the desired Memo text (or a period as placeholder, ".", to prevent unwanted text) and checkmark the "Lock this entry …" box. You now have a model entry for Amazon transactions which, being locked, will not be changed when you edit new Amazon transactions.
@PfunnyGuy Something doesn't add up. If Quicken is picking a transaction in the register that is a "match" and the downloaded transaction and the matched existing transaction will be merged. There are settings to have Quicken pick a category without using the memorized payee list, Edit → Preferences → Downloaded transactions → Automatically categorize transactions, but there is no such thing for the memo.
That leads me to believe that there is something corrupted in the data file. People have lost their memorized payee lists at times. It is possible that something like that happened, and it is just not displaying, but is still there. Another possibility is that you are using Sync to Mobile/Web and it is somehow messing up things. I would try:
File → Validate and Repair File → select Validate File → OK
Check if the Memorized Payee list is back or not, and check the functionality of downloading. You might also try:
File → Copy File or Backup → Create a copy or template → Next… But note that for this one it disconnects all the online services and you will have to reconnect them.
Also @UKR your suggestion is based on a wrong assumption that has been proven wrong in the past. You can't put a period in the memo of a memorized payee to override what is being downloaded. I discovered long ago that the only time that doing this overrides what is downloaded is if you use the memorized payee to make an transaction entry in the register. In other words, Quicken will always use the downloaded memo over what is in the memorized payee even if it uses that memorized payee to fill in the category. But it will not override a memo that is in a matching existing transaction in the register.
Also I will point out that one can disable Quicken using the downloaded memo at all by going to the Account Details → Online Service tab, and selecting Don't update memo when downloading.