Operating system: Ubuntu Studio 18.04.1, Linux 5.4.0-42-lowlatency kernel, 64-bit
Graphics card: NVidia 1050TI
Worked: 2.83 LTS
Short description of error
When one strip ends and another begins at the currently positioned frame in the timeline, using bpy.ops.sequencer.select_side_of_frame() (either side='LEFT' or side='right') results in both strips being selected. In 2.83 LTS only strips on the requested side were selected.
Exact steps for others to reproduce the error
Put two strips into the VSE. Snap the playhead to the end of one. Select the other and snap it to the current frame. Deselect all, then select side of frame (either left or right). Both strips will be selected. See attached screencap for demonstration.