This definitely looks like a known bug. See those vertical lines on your graph? That could be a ride (or two or three) that has a crazy high TSS and is messing with the scale of your overall TSS graph.
To fix it, you can either exclude the TSS from the(se) outlying ride(s), or if you had a really low FTP on a past ride (say, 20), change your FTP history for those rides with high TSS, since they could be affected by that metric being off.
You can also shoot us an email at firstname.lastname@example.org. Our Support Team would be happy to take a deeper look into that for you.