That is correct. I do not recommend dealing with an agent at all.
In Thailand, agents are unregulated, unlicensed, and most agents bribe an immigration officer. When a foreigner uses an agent inside Thailand, the following risks exist:
1. The agent loses the passport.
2. The agent may have a copy of the visa stamps and stamp the passport themselves illegally.
3. The agent may decide not to support the customer because they lost the contact within the immigration officer who issues the extension of stay.
4. The agent may steal the passport, other documents, or money from a foreigner’s bank account.
5. The extension of stay that was issued may be retroactively voided if the immigration officer is audited and falsified documents are discovered.
6. Once a foreigner starts using an agent, it is almost impossible to stop using the agent without leaving the country.
All of these have happened before; and I have proof either from Bangkok Post articles, or Facebook posts