I think there's a bug in the split line at points
tool. I used it recently (in QGIS 3.4) and got a similar result, with lines that extended far off the screen. I haven't gotten around to making a bug report yet. If you make one, please provide a link in a comment (or edit it into your question, if you don't have the commenting privilege yet).
Here's a workaround using the similar tool, split lines with lines
.
Convert your points into short line segments with the geometry by expression
tool. Use an expression like this:
make_line($geometry, translate( $geometry,1,1))
Use the SAGA tool split lines with lines
-or- the QGIS tool split with lines
. Both worked for me, but the QGIS tool was significantly faster.
split line at points
tool. I used it recently (in QGIS 3.4) and got a similar result, with lines that extended far off the screen.