I worked with Peter a couple of months ago to get fwupdate working on
arm64 (and with FMP capsules in general). One significant difference
is that fwupdate no longer adds a capsule header if the capsule
already has one, which sounds like an improvement in any case.
These changes are all here
but have not turned up in the debian packaged version yet. We will
shortly see more arm64 systems that require these changes, so please
consider pulling them.