Simple reproducer:

$ echo "echo foobar" | bash -t

should output:

foobar

But instead it produces no output. Several experiments with options like '-e', '-x' and '-v' were done.

First guess, but untested is that replacing eval.c:reader_loop() block:

if (just_one_command)
EOF_Reached = EOF;

with:

if (just_one_command)

might correct the issue.

Diff with bash-5, where it works, shows a call to 'reset_readahead_token ();'
but this does not exist in bash-4. This should not be the cause also.