WahWah: Incorrect preview with multiple tracks

**Describe the bug**
Real-time preview of WahWah effect may sound different to the effect when applied.

**To Reproduce**
Steps to reproduce the behavior:
1. Generate a 30 second tone, 1000 Hz 0.3 amplitude.
2. Duplicate the track
3. "Split Delete" the first 0.333 seconds of the second track
4. Select All
5. Launch the WahWah effect
6. With default settings, click "Play"
7. Click "Apply" and close the WahWah effect
8. Play

**Actual behavior**
Observe that preview in step 6 sounds different from playback in step 8.

**Expected behavior**
The applied effect to sound the same as the preview.

**Screenshots**
If applicable, add screenshots/video to help explain your problem.

**Additional information (please complete the following information):**
- OS: Xubuntu 20.04 (but expect this to be all platforms)
- Version Audacity 3.2.0 alpha (and earlier)
- (if relevant) Sound Device [e.g. Windows WASAPI, USB Sound card]

**Additional context**
Add any other context about the problem here.