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.
Inside playdate.update() print getFps(). After pausing and resuming the value will be off.
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?