incorrect diff in MPs: modified file shown as entirely new

In this MP: https://code.launchpad.net/~ahasenack/ubuntu/+source/ubuntu-advantage-tools/+git/ubuntu-advantage-tools/+merge/337213

Some modified files are being shown as entirely new ones.

I'll grab debian/rules for an example.

This is the real change:
$ git diff bionic-version-14..ubuntu/devel -- debian/rules
diff --git a/debian/rules b/debian/rules
index 0cd4e1d..b7e0591 100755
--- a/debian/rules
+++ b/debian/rules
@@ -3,4 +3,4 @@
        dh $@
 override_dh_auto_test:
        python3 -m unittest discover tests
- python3 /usr/bin/flake8 tests
+ flake8 tests

This is what LP is showing me in that MP:
diff --git a/debian/rules b/debian/rules
new file mode 100755
index 0000000..0cd4e1d
--- /dev/null
+++ b/debian/rules
@@ -0,0 +1,6 @@
+#!/usr/bin/make -f
+%:
+ dh $@
+override_dh_auto_test:
+ python3 -m unittest discover tests
+ python3 /usr/bin/flake8 tests

https://git.launchpad.net/~usd-import-team/ubuntu/+source/ubuntu-advantage-tools/tree/debian/rules right now agrees that debian/rules exists and has this content:
blob: b7e0591519313e7a715a89169c3fd162bc02308f (plain)
#!/usr/bin/make -f
%:
 dh $@
override_dh_auto_test:
 python3 -m unittest discover tests
 flake8 tests