When using certain Xilinx IPs, the burst type FIXED is not supported. In practice, they are not used (len=0) anyways.
AXI SmartConnect product guide (PG247):
SmartConnect does not support FIXED type bursts. Any FIXED burst transaction received at the SmartConnect SI is blocked and a DECERR response is returned to the master.
When using certain Xilinx IPs, the burst type FIXED is not supported. In practice, they are not used (len=0) anyways.
AXI SmartConnect product guide (PG247):