I'm from Mississauga and I can tell you there have been a few instances where I've boarded a bus with an out of service reader. Luckily the driver always lets us on if we show the card (MiWay buses are front-door load only).
It was said on a different forum that #4417 is having issues and is currently reserved for training.
https://cptdb.ca/topic/9298-missing-in-action-buses/?page=3
On the sammdot tracker you'll see #4417 spending a lot of time in the garage. It was used a lot up until about a month ago.
http://ttca.sammdot.ca/ttc-track-multi.html?4400-4603