This answer record discusses the UltraScale GTH and GTY transceivers MGTAVTT negative current that happens only under some startup conditions.
Under a power sequencing condition where VMGTAVTT and VMGTVCCAUX are less than or equal to 0.63V and VMGTAVCC is at its nominal level, a current as high as 35mA per transceiver could be sourced from the MGTAVTT pins on the FPGA device.
In this condition, the source of the current is the MGTAVCC power supply. This condition does not affect the reliability of the device.
The additional current from MGTAVCC to MGTAVTT is accounted for in XPE. It is not recommended to deviate from the power-on/off power supply sequencing in the data sheets (DS892 and DS893).