The issue only occurs when using [fcopy] and mk4vfs channels without direct mode.

Simple test:
<verbatim>
vfs::mk4::Mount test.mk /TEST
set fd [open
fconfigure $fd -translation binary
set out [open w]
set ::fcopy_complete 0
proc fcopy_complete {args} { set ::fcopy_complete 1 }
fcopy $fd $out -command fcopy_complete
after 3000
update
puts "FCOPY: $fcopy_complete"
</verbatim>