Reading through the Telethon documentation, this looks to be a known issue.
Telethon Docs

First and foremost, this is not a problem exclusive to Telethon. Any third-party library is prone to cause the accounts to appear banned. Even official applications can make Telegram ban an account under certain circumstances. Third-party libraries such as Telethon are a lot easier to use, and as such, they are misused to spam, which causes Telegram to learn certain patterns and ban suspicious activity.

It looks like the usage of Telethon flags the Telegram API anti-SPAM measures. For sensitive countries using a proxy might circumvent the ban. However, this use case does seem like it might be in breach of Telegram API TOS.

In the first instance, I would read through the Telegram API TOS to consider this use-case for Telethon.