Lastmod is not filled if date is read from frontmatter

The documentation says that lastmod falls back to the date variable if not set. However, I am asking hugo to read the date from the frontmatter instead (because I migrated from Jekyll), and I did this using

date  = [":filename", ":default"]

This works and I have the date, but lastmod is not set (returns the default value of 01 Jan 0001) for the posts even though the .Date parameter can be accessed in the templates. This problem disappears (i.e. the lastmod is set correctly if I explicitly add a frontmatter variable in the post). Is this the expected behaviour? This is not major for me but I am just surprised by this behaviour and took me sometime to notice the problem.