Physic based golf game - ball bouncing off the connection of colliders on flat surface

This is a common problem and there isn't really a way to fix it the way you have it currently. The best solution is to use a single MeshCollider for the whole course.

If you build your courses with modular pieces you would need to merge the seperate MeshColliders into a single one when the level initializes. We did something similar for a racing game that was a bit like TrackMania. If the placement of the modular pices isn't perfect you may want to do an additional procedural "vertex snapping" while merging two mesh colliders. That means you just search for two vertices that are very close together but not identical and set both to the same position (for example to the mean of the two positions: (p0 + p1) / 2)