In some situations when using the followLine vendor option the label is rendered on the left top corner and is barely visible. I attached two print-screens one that shows the bug and another one that shows the expected output. Note that this bug only happens when moving the line to a very specific position, I also manage to reproduce this error with others data-sets.
I also attached the dataset I used and the style. To reproduce it just zoom on the lines and move the map, or you can give a try to this WMS request:
I named the layer follow_lines, used it.geosolutions workspace and used the follow_lines style (attached).