What i have seen is controllers using the wrong callsign because they only see the 3 letter identifier?
Then sometimes the pilots use it as well because hey that's what ATC seems to be expecting.
Alternative theory: They used their name in Bulgarian instead of English? Then the controller does not understand and just uses what may be correct and sound similar.
Especially with the number 5533 after the callsign controllers and pilots might not care too much about the callsign as it's already clear who is meant.
Burgas Air with a bulgarian touch making it Burgash?
Anyone's guess unless you have some tapes to listen to.
Even then it might be anyone's guess
