I wasn't sure how gen 1/2 handles the removal of a Pokemon after a trade. In Gen 3 they just remove the species ID but leave the rest of the data, I hoped something similar for gen 1/2.
So I opened your save in a hex editor and searched for the nickname. I found it still in slot 6 of the party, but the counter was at 5. After setting it to 6 I was able to see your Charizard in PKHeX.
There's another way. Poke J's PKX Extractor scans the full file for any Pokemon data. With this you could find your Charizard as well. However for gen 1/2 the OT and nickname need to be manually fixed.