I don‘t know the details but AY does this indeed for many routes and it changes from time to time. For example FIN6KP became FIN5XB for AY1514.

What I do know is that suffixes may be added to avoid confusion with similar flights / flight numbers (mainly for communication purposes on frequency). If the letters follow a specific pattern, I don‘t know. It‘s not impossible.

It‘s quite common across carriers in Europe.

P.S.: I dug a bit further. Looks like the alphanumerical flight numbers are assigned randomly by a software to avoid similar sounding callsigns on a given time on a given frequency.