seconds behind master in slave status refers to its master's master if GTID enabled

In a classic replication setup A -> B -> C , when I run "show slave status" on C, I expect Seconds_Behind_Master to report the delay in replication in respect to its master B.

If GTID are enabled however, it reports delay in respect to A, when in reality C is caught up with B - in fact, disabling GTIDs makes SBM correctly report zero.

I noticed that Gtid_IO_Pos has the GTID from A listed before the one from B. Could be the source of the issue, although I think that slave should search for B's GTID specifically and not just use the first in the list.

The two GTIDs have different domain ID, one more reason for C to ignore the one from A; unfortunately, it doesn't.