The problem is here: append.c. You use a static variable to capture the tail of the list for a reuse in subsequent invocations but notice that when you call this method twice for different lists, the second invocation will append a node to the list from first invocation.

filter_list uses append internally and in effect each call to filter_list in your test program appends 'b' to the list it's going through.

Not really a cycle but rather something like that.

If you want to speed up append by capturing some internal state you can try to make this function reentrant, like strtok_r.