How do I achieve 1 byte only communication on FIFOed Avalon Uart?

Hello,

I am trying to use the latest version of this FIFOed Avalon Uart component:

https://forums.intel.com/s/createarticlepage?language=en_US&articleid=a3g0P0000005RTHQA2&artTopicId=0TO0P000000MWKBWA4&action=view

to setup an UART slave device on my Cyclone V. Everything seems working great when I receive more than 3 bytes but I found weird behaviors when the communication is about one or two bytes. Here the NIOS interrupt service routine triggers as soon as the byte is received but if I check the valued stored in the RX FIFO I always find "0x0000" (always zero).

Any idea?

In attach the IP settings.