Why is 'multipath -r' reporting "path is down" messages when the paths are active?

Issue

The multipath -r command shows many "path is down" messages, even though paths are known to be active.

 mpath248 (231d980035423c53967555a1424678555) 
[size=10G][features=1
 queue_if_no_path][hwhandler=0][n/a]
\_ round-robin 0 
[prio=200][undef]
 \_ 2:0:3:1  sdaa 65:161 [active][ready]
 \_ 2:0:4:1  sdam 66:83  [active][ready]
 \_ 3:0:2:1  sdbk 67:133 [active][ready]
 \_ 3:0:3:1  sdci 69:63  [active][ready]
\_round-robin 0 [prio=30][undef]
 \_ 3:0:2:1  sday 67:31  [active][ready]
 \_ 3:0:4:1  sdbw 68:130 [active][ready]
 \_ 6:0:0:1  sdc  8:23   [active][ready]
 \_ 6:0:4:1  sdo  8:223  [active][ready]
sdba: checker msg is "directio checker reports path is down"
sdaf: checker msg is "directio checker reports path is down"
sdax: checker msg is "directio checker reports path is down"
sdbc: checker msg is "directio checker reports path is down"
sdbg: checker msg is "directio checker reports path is down"
sdcn: checker msg is "directio checker reports path is down"
sde: checker msg is "directio checker reports path is down"