Unable to align vertices along a custom orientation

I’ve already asked this question on stackexchange, but nobody answered. I’m just wondering if it’s a bug or expected result. I thought that by scaling vertices by zero with custom orientation I would get perfectly aligned vertices along this orientaiton, but instead I got curved line:

This is what I expected to get ( this time I scaled these vertices along global axis and then rotated them):

Custom orientation doesn’t align the view. It’s already scaled to 0.

Apparently my custom orientation’s X axis wasn’t parallel to the global’s X axis. I thought they were parallel. My mistake, question closed.