Hmmm I just checked Garmin Connect as well and was presented with the following graphic on my training mix which might be another clue on why TSS is so high. Alot of my rides are in the Anaerobic area as I was completing the short Power build program so that would make sense but I am way outside the optimum range and it is continuing to grow.
Based on what people have said, I might scale back the high intensity sessions this week then attempt to do an outdoor FTP test with the Garmin to see if there are any discrepancies between indoor and outdoor. If there isn’t I probably need to scale back the outside rides a little if I want to get faster, if there is then I am not sure what I will do as it might be a little too hard to have an indoor FTP and outdoor FTP on the same device. 