To avoid meditation sessions being logged as Exercise, tap the Activity tab in the Timefully app on your iPhone. Tap Settings and deselect the option "Register As Exercise".
Important note: Until Timefully has been granted special permission from Apple to track heart rate during meditation sessions, unchecking this option will often cause the heart rate to not be measured during meditation with the Apple Watch. As soon as Apple grants us this permission, the heart rate tracking will work also with "Register As Exercise" disabled. We will update our users as soon as this works as expected, but at the moment there is unfortunately nothing we can do to fix this issue.