Bug description

Discovered while testing #9107

Steps to reproduce

1. Load a clip into AU3
2. Export as WAV, 16 bit, 44100 Hz
3. Load the same clip into AU4
4. Export as WAV, 16 bit, 44100 Hz
5. Load these two exported clips into AU3
6. Invert one of them
7. Mix and render
8. Measure RMS of the resulting track

Expected behavior

-inf dB

Actual behavior

Around -81 dB, which points out that the result that AU4 produces isn't a bit-perfect match with AU3.

Audacity Version

master

Operating system

macOS

Additional context

No response