If you do `arc diff` and suppress linting (say), and you type in an excuse, the excuse shows up on the differential review page, great. If you then run `arc diff` again -- even `arc diff --verbatim` -- and suppress linting again, the new excuse overwrites the old one.
At least some people here had the expectation that the new excuse would be appended to the old excuse, rather than replacing it, due to the tendency of phabricator to do appending in other situations.
One solution would be to do appending for excuses; an equally good one (from our point of view) would just be to document the behavior better somehow. Or, perhaps, if there's an existing excuse, have arc offer that as the default when asking for a new excuse.