Why is a Gluster mount point returning 'Transport endpoint is not connected' after a rolling reboot of the Gluster nodes?

Issue

• After correctly restarting Gluster nodes / pods one by one, a Gluster file system mount point is returning Transport endpoint is not connected:

  Filesystem                             Size  Used Avail Use% Mounted on
  192.168.1.1:glusterfs-registry-volume  5.0G  1.1G  4.0G  22% /registry
  
  df -h /registry
  df: '/registry': Transport endpoint is not connected
  

• However, the matching Gluster volume is online, with all its bricks up and there's no pending healing:

  Status of volume: glusterfs-registry-volume
  Gluster process                             TCP Port  RDMA Port  Online  Pid
   ------------------------------------------------------------------------------
  Brick 192.168.1.1:/var/lib/heketi/mounts/vg
  _192cd678d5d6bb3ebc5111f906f576c4/brick_0cc
  a0b2f00e4fa82dca7859550c5fc66/brick         49154     0          Y             359
  Brick 192.168.1.2:/var/lib/heketi/mounts/vg
  _248d829a7538a61c5e7d43c8325652e8/brick_13f
  f85e4bf0362bc2218600c860dc229/brick         49152     0          Y       358
  Brick 192.168.1.3:/var/lib/heketi/mounts/v
  g_41c604c20fe4c4de54b9b7b120448756/brick_0e
  3d063b30e02fc5dc03eafb3e65f06b/brick        49152     0          Y       358
  Self-heal Daemon on localhost               N/A       N/A        Y       318
  Self-heal Daemon on 192.168.1.2             N/A       N/A        Y       315
  Self-heal Daemon on 192.168.1.3             N/A       N/A        Y       329
  

• Why is this issue occurring? How to fix it?