Function definitions within non-forked subshells (including command substitutions) are silently ignored if a function by the same name exists in the main shell, so the wrong code is executed. unset -f is also silently ignored. Test script:

foo() { echo WRONG; }
(foo() { echo ok; } && foo && unset -f foo && foo)

Output:

WRONG
WRONG

Forcing the subshell to be forked (by making it a background job, or by including a redirection within a command substitution) is an effective workaround.

Silently executing the wrong code under any circumstances is a bad bug, but it seems to have been in ksh93 since 1993...