AREF above operating voltage of the microcontroller
Indeed this information will be in the datasheet, but since it might be difficult to find I'll help a bit:

The datasheet will state that the voltages at the uC's inputs/outputs cannot exceed the voltage on the VDD pins.

The reason for this is the ESD protection diodes. As soon as the voltage at AREF becomes higher than VDD + 0.5 V the AREF's ESD diode will start conducting.

So if you connect 5 V to AREF, the VDD supply of the uC will be pulled up to about 4.4 V ! But you supplied 3.3 V via an LDO. Yes, then the LDO will do nothing.

If you want a 5 V input range either power the uC with 5 V

or

use a voltage divider to lower the input voltage to the ADC.

To learn more about what issues the ESD diodes can cause, watch this EEVBlog video