WSREP_LAST_WRITTEN_GTID returns incorrect result

Querying "SELECT WSREP_LAST_WRITTEN_GTID()" at the same time with different connections on one single node yields different results.

The GTIDs returned are way of from each other:
Connection 1: 0-1-3311413119
Connection 2: 0-1-3311399223
Connection 3: 0-1-3311430041
Connection 4: 0-1-3311374352

Lowest GTID returned was from about 10 minutes before.

Even 15 minutes later WSREP_LAST_WRITTEN_GTID returned the GTID 0-1-3311401500 on a fresh new connection with one single write, which is still way lower then those from earlier.

To reproduce:
1. Create 2 or more connections
2. Run a simple write query
3. SELECT WSREP_LAST_WRITTEN_GTID()
4. Compare the results

Expected behaviour should be a steady increasing GTID.