Machaca, great image work as always.
PJ2, I presume your source for ATA section codes is the pilot's MEL? If so, did you type that from the onboard MEL?
That's the POI approved version specific to a particular airline, is it not?; an abbreviation of the Airbus Master MEL. If so, it's a legal dispatch reference, and not a maintenance trouble shooting reference. Unless I am mistaken, it only addresses dispatch concerns, such as Air conditioning that your POI permitted you to defer, whereas, the full volume derived from the type certificate data sheets in France may be very different and the same numbered section may include both air conditioning and pressurization items (just as an example) even though the section is called "Air Conditioning" on both.
Your posts are extremely good, but your list starts at section 5, and omits sec 13-19, 33-44, 50, 58-70 etc if those sections exist.
To draw conclusions from your list seems wrong to me. We should be using the specific ATA section and item number from the actual maintenance volume. (As an A&P) this is what I was taught.
For example you said:
ATA 21 is the Air Conditioning Chapter. This last message is indicated as an ADVISORY level message, (appropriate text is pulsing bright/dim green on the lower ECAM. However, the "Typ" (type) of message is "WRN", (Warn). I have no means to interpret this. There is no indication of cabin pressure loss. That is all this message means - that there is no message.
I'm not convinced yet that "there is no message" as you said.