What your experiencing happens all of the time. If I had to guess you have a brown cover and your spa catches sun all day. Growing up at my parents house their spa sits under direct sunlight and during the summer months it isn’t all that uncommon to see the temp rise that much due to the sunlight. The spa probably isn’t calling for heat (check at different times through out the day to see if there is a "sun" icon being displayed) if that icon isn’t on, then the heat gain is just due to the warm days. As for ways to eliminate the heating problem you could leave the cover open on the warm days to cut down on the heat buildup.
Hope this helps.
