Dimmable LED goes haywire when other devices plugged into switched outlet

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A couple other findings, may be important:

1. There seems to be ~10V between neutral and ground at the outlets (and at the switch on the line side). This seems high? Does that mean a floating ground, and would that be the problem (or just a different problem)?
2. When I measure the line voltage going into the switch, it's a steady 120V. When I measure between neutral and the load side, it's ~100V for a few seconds before the multimeter reads ~50V for a second, then back to ~100V. It does this repeatedly, despite being at "full brightness".

Another update. I bought a new Leviton smart dimmer and new bulbs to try that. Exact same behavior, even with a different bulb. BUT the Leviton app has a setting called "control timing" that I can set to "extended" and this completely eliminated the issue. I assume this is just changing the PWM frequency. Awesome!