I agree with you. Sometimes, very much so. But.. sometimes I'd be annoyed if it didn't reset - like if I dropped the kids off at school, ran home and got water, then went on a 2 hour trip. I'd be upset that the school portion was lumped in. I think it's hard for the car to know, so I suppose all we can do is use Trip A for the times we want it aggregated, and use the automated trip for the pretty-close-to-majority of times when it works fine.
The other idea would be some kind of selection that says "lump my last trip with this one". But I don't think Tesla's going to prioritize that right now.
Or they could always group when X minutes apart (whatever value, though 15 is probably good) and we can use the Reset button in Settings when we have a scenario like yours.