Correct me if I’m wrong, but if you forgot a mortal sin in your last confession, the absolution is still valid, so it would be logical to continue receiving communion until the next opportunity you can have a confession and can mention the mortal sin you had forgotten.
A hypothetical situation:
At my parish church they have confessions everyday at the 6PM mass but not at the 7AM mass which is the only one I can attend during the week and in all Saturday and Sunday masses. If I forget a mortal sin at a Sunday confession, I will continue receiving communion all week and mention the mortal sin I forgot at confession the following Saturday.
If you commit a mortal sin after your last confession and then forgot and remember it after the host is on your hand, that’s a different story. I would just consume it, the reason being that it’s already on your hand, so if you try to return it you could make people around you including the priest very unconfortable. I would mention that on my next confession though and not receive again until after it.