The definition of left and of right alignment currently both say:

If the property’s axis is not parallel with the inline axis, this value behaves as start.

It seems less than ideal for both of them to fall back to start, since there may be an expectation that they end up on opposite sides.

I think one of them should probably fall back to end. I'm inclined to say it should be right, given that line-left is equivalent to (see mappings) inline-start whenever direction is ltr. This is for two reasons:

• I believe direction: ltr is more widely used (on the Web, and as a share of world population)
• I suspect the main use case for such fallback might be flipping the writing-mode of CJK content between vertical and horizontal, where I suspect direction: rtl to be particularly rarely used.