All your questions can find a reply on the power-up procedure chapter in the device datasheet (pag.24).
If you respect it, you should not experience any problem. Is IRQ1 interrupt pin pulled low at the end of the transition to PSM0?
Please note that the power-up timing requirements and voltage level are reported in figure 35 and there are no timing restrictions on the power-down procedure.
Moreover, PM0 and PM1 define the power mode of the device and, during power-up, you need to ensure that PM0=1 and PM1=0 (normal power mode).