Anaerobic training effect calculations for indoor bike

Hello!

I am wondering if anyone has a similar issue with Garmin’s anaerobic effect calculations regarding indoor biking. I bike on an old schwinn stationary bike with no connectivity options. After hard “anaerobic” sessions where I reach my maximum HR zone for short intervals. Although I reach this zone within various intervals, my anaerobic training effect remains close to zero. Does this have something to do with a lack of connectivity for power output and speed on the bike? Or am I actually just not getting any anerobic training effect during my sessions?

Thank You!