With the xFD, can you identify exactly where the antenna is on that reader?
Maybe draw the trace with a pencil or whiteboard marker.
Whats the voltage range on that reader?
What voltage are you using?
What mode is your ug4 in?
MF1k 4 Byte?
Can you copy the FlexM1 that works to it?
Can you do a PM3 hf search or TagInfo scan and share it
Also, well done getting a reader for your testing