And then returns back to the overworld.
And it does happen at any speed. The screenshot is of my standing on an invisible fence, and of the log trying desperately trying to put the player on the ground.
http://prntscr.com/qppqnm
The log snippet here shows what happened to cause the fences to turn invisible, and links it to
[00:51:42] [main/INFO]: [Portal] Client Changed Dimension from minecraft:overworld to simplevoidworld:void time: 1022
[00:51:42] [main/INFO]: [Portal] Portal Number Near Player Now0
[00:51:42] [Server thread/INFO]: [Portal] AltimusX teleported from minecraft:overworld class_2338{x=-526, y=254, z=-722} to simplevoidworld:void class_2338{x=-526, y=-2, z=-722}
[00:52:32] [main/ERROR]: [Portal] The Player is Moving Too Fast!
[00:52:35] [main/INFO]: [Portal] Client Changed Dimension from simplevoidworld:void to minecraft:overworld time: 2083
[00:52:35] [main/INFO]: [Portal] Portal Number Near Player Now0
[00:52:35] [Server thread/INFO]: [Portal] AltimusX teleported from simplevoidworld:void class_2338{x=-526, y=-5, z=-722} to minecraft:overworld class_2338{x=-526, y=251, z=-722}
[00:52:35] [main/ERROR]: [Portal] The Player is Moving Too Fast!
[00:52:36] [main/ERROR]: [Portal] The Player is Moving Too Fast!
[00:52:37] [main/ERROR]: [Portal] The Player is Moving Too Fast!
[00:52:42] [Server thread/INFO]: [Portal] request teleport AltimusX in minecraft:overworld to -524.000573154433 67.0 -719.6922913376726
[00:52:43] [Server thread/INFO]: [Portal] request teleport AltimusX in minecraft:overworld to -523.9483578862918 67.0 -719.609360242799
[00:52:43] [Server thread/INFO]: [Portal] request teleport AltimusX in minecraft:overworld to -523.9483578862918 67.0 -719.609360242799
[00:52:43] [Server thread/INFO]: [Portal] request teleport AltimusX in minecraft:overworld to -523.938841304021 67.0 -719.5922256380607
[00:52:43] [Server thread/INFO]: [Portal] request teleport AltimusX in minecraft:overworld to -523.938841304021 67.0 -719.5922256380607
[00:52:43] [Server thread/INFO]: [Portal] request teleport AltimusX in minecraft:overworld to -523.9311674473319 67.0 -719.5741903409812
[00:52:43] [Server thread/INFO]: [Portal] request teleport AltimusX in minecraft:overworld to -523.9311674473319 67.0 -719.5741903409812