Diffing submodule does not yield complete logs for merge commits

I am attempting to diff a submodule modified in my working copy and
the only difference is a merge commit. However, I do not get the
"full" range of commits introduced by the merge commit when I diff it:

$ git diff --submodule=log Core
Submodule Core 8b4ec60..def2f3b:
  > Merge remote-tracking branch 'origin/master-ah3k'

However if I go inside my submodule and run `git log` by hand, I get
more information about the TRUE commits introduced:

$ git log --oneline 8b4ec60..def2f3b
def2f3b Merge remote-tracking branch 'origin/master-ah3k'
015c961 Remove log spam in FontManager
7713ba1 Update third party submodule to latest
10aac78 Merge pull request #9 in FE/core from
feature/FE-1348-selecting-continue-on-zero-balance to master-ah3k
287882f FE-1376 Nedd to remain in check detail screen when selecting
donation after SBI
a5a6bed Do not overwrite the current check# within loop
dfb8547 Adding list of checks to CRspChecks before saving
1be280a FE-1354: Guest logged out in specific multiple check scenario
de06d5a [FE-1348] Fix PATT exit while checks still open

It's almost as if the `git diff --submodule=log` approach is passing
in --first-parent to git log, which would exclude commits in the range
that I'm seeing when I run git log manually.

Is this by design? Is there a way to enable the full log history with
`git diff` on a submodule?