Why does my 530 want to update my FTP with a number less than my FTP?

Because it's detecting an FTP number different of what you've setup manually. Zwift or other apps use a different method to calculate FTP than Garmin and if you're using a power meter while riding outside with your garmin and a smart trainer indoors.....then there are differences between both power readings.

Basically...Garmin will always try to prioritize its own way of detecting/reading metrics vs the data you manually input

Forgot to say...if you want this message to stop, just disable the option to automatically detect FTP on your Edge 530, then it will just take the number you say your FTP is regardless of what it's measuring.

"Garmin will always try to prioritize..." is exactly the problem. I get that it's running algorithms in the background, and it's great that it does that. The issue is that if I tell it a number, and it "detects" a number that's lower than the number I give it, it should just be silent.

The only condition where it shouldn't is if I explicitly do a FTP on the device. In every other scenario, it can only reliably guess upwards. Any downward guess is going to be wrong because it doesn't know the reason why the power dropped.

Traffic, stop lights, stop signs, not doing an FTP test, running out of rabbits... the list of reasons why I might let off the power short of whatever algorithm it's using to guess are myriad.

It will always be wrong when guessing down. So it shouldn't.

My goal in raising this is to provide, what I think is useful, feedback to the product team.

Andrew Van Til said:

Any downward guess is going to be wrong because it doesn't know the reason why the power dropped.

This isn't true.  FTP can decrease for genuine reasons not just traffic lights. Your fitness may have dropped or the previous measurement may have been wrong.  FTP is also sensitive to your posture, temperature, diet, health etc. 

You already have the ability to disable Garmin's measurements. Also, it is the trend over time that is important not an individual measurement.  If you accept a lower measurement that is wrong in your opinion, then it should correct itself over time. Thus, the way Garmin has chosen to do it is better for most people than potentially flattering yourself with upwards-only adjustments.

Yes, it's true that you can detrain. But, there's no way for the device to accurately know that unless you do an FTP test. Unless you signal your intent, it will be wrong because it doesn't know why your behavior is what it is. No amount of apologizing on Garmin's behalf changes that.

Blinders aren't really my thing.

You can completely disable this function as others said, or you can discard its guess if you don't agree with it. Garmin doesn't do this "blindly". When it finds a long enough stretch of high effort activity, so that it thinks it can give you a decent guess for your FTP, it will offers it. It knows it's not perfect, thus the option to accept or discard it.

I like the prompt and find it useful, as I definitely de-train from time to time and do proper FTP tests rarely and this can keep my Power Zones in line. My gripe with Garmin is usually that I don't have enough option but in this case they really are giving us all the options. You're suggesting to remove a function that some find useful.

I appreciate the desire to tell me I'm holding it wrong. But, there is frankly absolutely no way it can accurately guess down without knowing your intent. Until there's an intent sensor, only telling it you are doing an FTP test can capture that signal.