As stated, that is from blowback.
Normally, casing expands with pressure to seal the chamber, only to shrink slightly when pressure is relieved.
Small soot, powder, chamber settings may allow some blowback until the casing seals.
If it appears to be too much blowback, then get with the smith and use a set of go/no-go checks.
Most of the time, it is of no problem to the shooter.
"A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."
Life Member NRA
Life Member NAHC
Former President of the ECPT (Eifel Combat Pistol Team)