The TPS debug chart only shows a fixed "20 TPS"

This is a misreported issue, please ignore it and close it if you can, thanks.

 

The TPS debug chart should show real-time TPS, but now it only shows a fixed "20 TPS".

What I expected to happen was...:
Show real-time TPS on the TPS debug chart.

What actually happened was...:
Only shows a fixed "20 TPS".

Steps to Reproduce:
1. Open any world.
2. Press F3+2.
3. Look at the bottom right.