The internal switch which switches the backup-domain part of the chip between VBAT and VDD is controlled from the internal reset circuit, so if you disable the internal reset, this switch presumably may flip randomly, and as it's not anticipated that it is switched to VBAT with VDD present, it may result in larger parts of the chip being incorrectly powered from VBAT, overloading the internal switch.
Whatever, this is just a speculation - whatever the reason is why this configuration is prohibited by the datasheet, you should simply not use it.
JW
