After three weeks' employment, I found that I was the only one in this department's online recruitment. What happened?

When you resigned, your leader had blacklisted you. Unless you are a skilled worker, the company can't operate without you. Except that you are irreplaceable. The leaders left you here just to stabilize you and give them time to recruit people to replace you. When your replacement is in place, you should, hen, leave here. When you are going to resign, you must keep high spirits.