Due to a problem in the Quartus® Prime Pro Edition Software version 23.4, the p<n>_ss_app_st_tx_ready signal will go to X (undefined) in simulation of 40GbE or 50GbE ports when the Enable Preamble Passthrough parameter is selected on Agilex™ 7 F-Tile variants of the Ethernet Subsystem FPGA IP.
To workaround this problem in simulation, do not select the Enable Preamble Passthrough parameter for 40GbE or 50GbE ports.
This problem has been fixed in version 24.1 of the Quartus® Prime Pro Edition Software.