It's not a problem in the sense that you can "fix" it. If you try really hard you can train yourself to notice when you jump mental tracks then possibly steer yourself back to the main line. At the same time even if you have if it's a topic you're really excited about? pfffff good fucking luck. Other external factors can also make it next to impossible to do anything about.

That's just how our brains work.

Honestly some of the best conversations I've had were with my best friend who also has ADHD. Good fucking luck following those conversations if you aren't neurospicy. We just embrace that we are enjoying the conversation so it does matter if the conversation started about D&D and 10 min later we are talking about tacos being to salty. -shrug- we will probably get back to D&D if we are exposed to the stimulus that started the conversation.