A new Young Sheldon season 6, episode 18 trailer confirms who George's mystery woman is.

Since the previously-released trailer didn't feature a front shot of Missy, there were some assumptions that George is talking to a different woman in the video. 

As confirmed by Sheldon himself, Young Sheldon season 6 is now set in the spring break of 1993, which means that his story about catching his father cheating is going to happen soon. 

Expect that The Big Bang Theory prequel will stick to tackling ongoing plot lines instead of introducing new ones that could distract from its more urgent storylines. That includes Mandy and Georgie's romance, as well as, Sheldon's surprising stint abroad in Germany. As for George's cheating storyline, it's likely that the bulk of it will happen in season 7.