Multiple rails I2C bus device from PMIC
The letter of the I2C spec says that the powered-down device is not supposed to drag down the I2C lines. In practice, this may not be the case, depending on how the chip designer implemented their I2C I/O pads.

I2C is supposed to be open-drain. However, sometimes designers will use a 'pseudo-open-drain', that is, a regular I/O pad that's wired to use output enable to make low / high-Z to form the I2C signal. The problem is, regular I/O pads have protection diodes that will kill the bus when the device is powered off.

Here's a clue: if the part has a Vi(h) spec on the I2C pads of, say, VccIO+0.5V, or they have no special statement about I2C vs. power-off, chances are there are protection diodes on the I2C pads that will be forward-biased by I2C on power-down.

The safest thing to do is to put an isolation switch between the powered-down I2C section and the main section, and disconnect the I2C domains from each other when the LDO is powered off.

You have options for the disconnect. You can use a level shifter with an enable, a signal switch (e.g., USB 2:1 mux like this one: http://www.ti.com/lit/ds/symlink/ts3usb221a.pdf) or even a pair of N-channel FETs for the cheapest solution.

If you can have a separate I2C bus for that peripheral and the PMIC, even better.