Due to a problem with the ASMI Parallel II Intel® FPGA IP, you will see this behavior if the read signal is asserted when the IP is stalling a new command with waitrequest high. The readdatavalid signal will be asserted one clock cycle after the read signal is asserted.
Because the IP is still busy, waitrequest stays high. The readdata bus from the IP is not valid.
To work around this problem, do not send the read command to the IP when the waitrequest is high.
Send the read command when waitrequest signal is deasserted.