It's normal in Teams channel, take a look at this documentation here:
Once the user enters their username and password in the prompt (hosted by the identity provider), they might be prompted to enter a validation code, depending on the channel. Some channels, such as Microsoft Teams, do not require the user to enter a validation code.
Thanks for your response! Though if the user is not required to enter the validation code, why am I transferred to the "sign-in" topic? And what can I do in order to continue with the conversation (since now I can't reach any other topic, the bot is not writing anything)?