This is the error when I use Algorithm: Cryptonight V4 (R)
[2019-04-14 12:49:03] GPU0[BUS:1] result rejected for job[15] [low difficulty share]
[2019-04-14 12:49:33] Pool sent a new job, block height 416327 [ID: 5ac7b313-96fd-4601-94a0-9e51cecb4439]
[2019-04-14 12:49:44] Pool sent a new job, block height 416327 [ID: 5ac7b313-96fd-4601-94a0-9e51cecb4439]
[2019-04-14 12:49:44] GPU1[BUS:2] result rejected for job[17] [low difficulty share]
I'm getting something similar with 1.8.3 and 18.6.1 drivers on a mixed RX 560 / 570 rig. Will be going back to 1.8.1 here shortly if the "low difficulty share" errors persist.
One other bug I noticed is that the min_rig_speed parameter seems to only be looking at one of the GPU's hashrate, rather the sum total.