When selected align left from the toolbar, it just removes any ql-align class.

'left' is missing from the whitelist, the align left button has no data-value attribute, so I guess this is sending a blot.format('align', false) (??)

This is based on an assumption that the default paragraph style is left aligned.

Many sites will use justify as the default alignment. The result in this case is that selecting left-aligned has no effect, the text remains justified.

Applying a left align style to a paragraph should do exactly that.

Solution:
Add 'left' to whitelist. Alignments should toggle if this is needed (ie applying center-align to a paragraph already center aligned should remove the align class).