The rule is that all players must be in their own half, ie both teams, except for one. Problem solved if your team come back to their own half and celebrate over the touchline, ie they are not then in their own half as they are not on the pitch!
Firslty, what 'potential' rule are you referring to? Also, one still of Trippier turning doesn't mean he instructed to wait in the oppo half. Looks more like in that photo that he is turning to celebrate with the bench.