The short answer is “I don’t know exactly”… but I have a good hunch. The T5577 chip works by configuring an “analog front end” which governs how the chip actually communicates data to the reader. The signal you are seeing on the graph is a direct result of this configuration setting. Clearly the chip is behaving and configured in a fundamentally different way from your other chips. This is probably the reason you can’t actually get it to perform a lf t5 detect properly… it’s not talking in a “language” that the detect code can interpret. To read more about the configuration, here is the data sheet;
How this happened, I’m not sure… but it’s probably a corrupted configuration block. This can happen from something called “tearing”…
Your best bet I think is to keep blasting the T5 with test mode recovery methods at it.
Follow the recovery steps… once you think you hit it plenty of times with recovery commands, clone an EM tag ID to it… you may still need to try blasting this several times at the chip to get it to pick it up… then do an lf search