My very first issue (and a different one)

Just took off from Halifax Intl (CYHZ) enroute to St. John's Intl (CYYT). Departure was the CYHZ3 Departure off RWY05, runway heading was 054. Upon takeoff I engaged the heading select and turned selector to 054 (it was on 053) and the aircraft did a complete 360 turn back to heading 054. Did I do something wrong? or was this a little bug I found. If anyone wants to try and recreate this my route was CYHZ3(RWY05) YHZ J575 LEXAK BURIN9 ILSRWY29. Like title says, I've never had a single issue with this aircraft until that happened, and who knows, maybe I forgot to do something or set something up improperly. Does anybody have any ideas? Sarah Guiney