Another scenario is for a Regular Sqn Ldr or Wg Cdr becoming a Reservist at Flt Lt rank. I was a serving Regular Wg Cdr that became a Sqn Ldr Full-Time Reserve (FTRS) - my No 1 uniform was re-ranked from Wg Cdr to Sqn Ldr. Oddly enough, I’m a Wg Cdr again now!
Alternatively, they have been given Acting rank, worn the Sqn Ldr rank for a a few months, and then never became Substantive before reverting back to Flt Lt.
I think those scenarios are more likely than someone putting up the rank before being Gazetted, in my humble opinion.