Blocking over the entirety of /filter doesn't necessarily imply that the callbacks themselves are not async. Perhaps we're arguing about terminology so I'll restate it: the callback is using a separate call chain, and rightly so, since it is a callback. You should not expect any commands to be executed in the same call chain unless you explicitly /call them yourself. I think that's fairly reasonable.
Note that this same behaviour occurs with local event identifiers, so it's not just an issue with $v1.
alias testfilter { echo -a filtering $1- $fulladdress }
on *:TEXT:*:?:echo -a starting with $fulladdress | filter -ffk test.txt testfilter
