Chips' temp is a chip reported by the chip itself, while the 4 sensors on the hashboard report the PCB temp, on the
T17 they can be found "near chip 9, chip 7, chip 22, chip 24.", their part number is "TMP451AIDQFR" and they look something like this.
I believe that stock firmware requires at least 2 of them to send temp-readings before mining is initialized, Vnish, on the other hand, requires at least 1 out of 4 IIRC, of course, you can get the miner to start without checking them if you would modify that piece of code, while that may be good for testing whether you really have 4 bad temp-sensors or just a loose chip/heatsink, it can set your miner on fire.
Since you mentioned 90c then it's chip temp, I assume if PCB temp is 90c then your chips would have probably melted, and as far as stock firmware, it works exactly like how you explained it, 1 chip out of all the chips across all boards goes above 90 and the miner goes to protection mode until restart, these settings are adjustable, most firmware including Vnish use 90c as a cut-off and honestly speaking even 90c is way too much for these 17 gears so perhaps thierry4wd should reduce it a bit, if it was S9, I am sure it would handle over 100 with no issue.