Is this deliberate?  I don't think I read anything in this thread
suggesting it; apologies if I missed something.

If it's a bug, maybe this is due to recent changes in xfaces.c?

    git log --oneline 1c29ba0340..21790e5473 -- src/xfaces.c 
    d4515f3cab Support ':extend' in faces defined by list of key/value pairs
    19aecd340b Fix face merging when some have :extend non-nil and some are inherited