The same thing happened to me today -- I selected Biking as the activity, but my battery died half way through the ride (I assume that caused the problem). I eventually noticed that Mountain Biking (or sometimes it shows up as Outdoor Biking) is in the list of "auto recognized activities". To find these, from the main page, click on "Exercise days", from that page, select the "kebab menu icon" (the three dots in the upper right hand corner), then select "Activity settings" and you'll see a list of "Auto Recognized Exercises".
I removed all of these from the old app because it kept thinking I was swimming when I folded laundry. Apparently they added them all back in against my will when I went through the migration (I was on a subway train at the time though, so maybe I missed a step in the migration dialogue in my hurry).
Also, the fact you list of recent activities is buried under "Exercise days" and if it's Sunday morning or early enough in the week that you haven't done anything, you won't realize you've come to the right place because nothing shows up until you click the arrow to go to the previous week says loads about the amount of thought that went into this design...
