I'm using a system where my players speed is based on frame rate so the player always moves smoothly. It gets multiplied by a Delta variable (1/playdate.getFps()). After pausing for roughly a second the Delta variable reports negative.
To reproduce:
Inside playdate.update() print getFps(). After pausing and resuming the value will be off.
The getFps() function is calculated over many frames, so it won't always be exactly accurate. It depends a lot on what your game is doing.
We could probably improve things a little by taking into account when the device is paused (I'll file an issue on that), but I wonder if using display.setRefreshRate() might be a more reliable option for what you're doing?