The Debian Expect maintainer tells me that this problem is present when libtcl8.6-8.6.4 is installed, but vanishes with 8.6.5 installed. So this is definitely not a bug in Expect (and can be closed), and probably doesn't need anything doing as it will be fixed in xenial.
The Debian Expect maintainer tells me that this problem is present when libtcl8.6-8.6.4 is installed, but vanishes with 8.6.5 installed. So this is definitely not a bug in Expect (and can be closed), and probably doesn't need anything doing as it will be fixed in xenial.