According to the IFAB q&a a player cannot be cautioned before kick off only red carded.
Q3: Why can a referee send off (RC) but not caution (YC) a player before kick-off?
Red card offences are serious and players who commit such offences should not play in
the match but it would become complicated if a caution (YC) could be given pre-match
as people would not know that a player has a caution (YC) when the match starts. It is
better that these offences are reported.
Double checked and law 5 supports this so either the player wasnt cautioned in the warm up or the referee made a mistake.
Q3: Why can a referee send off (RC) but not caution (YC) a player before kick-off?
Red card offences are serious and players who commit such offences should not play in
the match but it would become complicated if a caution (YC) could be given pre-match
as people would not know that a player has a caution (YC) when the match starts. It is
better that these offences are reported.
Double checked and law 5 supports this so either the player wasnt cautioned in the warm up or the referee made a mistake.
