How a singleton designed app effects tempdb

We have been experiencing some weird Tempdb growth that I haven't been able to figure out I had been trying to monitor what what happening in the tempdb dirong growth events and the data just seemed weird. So today using 3rd party software it snapshot the tempdb transactions and I noticed that the spids remained the same and the values were accumulating. I then realized that there was only 1 session per web server and the sessions and spid were over 6 hours old. I was under the impression we were using connection pooling, so I asked a dev. He said "optimized to use the same Session since it never changes its connection information.
This is a singleton by design"

So I am thinking to myself if the same session and spid is being used how does the tempdb manage the transactions? I have been googling and not really finding any information. I know, I am not finding helpful data because all of the data is cumulative.

Has anyone else experienced this? I always thought best practices was connection pooling and closing the connection as soon as possible for security.