Changes too close to the end of a line are not highlighted

Consider the following files/versions:

(Old)

Do you see
the seatbelt?

Please fasten
the seatbelt tightly.

What's a
seatbelt?

(New)

Do you see
the seat belt?

Please fasten
the seat belt tightly.

What's a
seat belt?

Only the middle delta is correctly highlighted:

This seems to happen if the region that should be highlighted has fewer than three characters after it (before the end of the line).

I expected "seatbelt"/"seat belt" to be highlighted in all three cases.