Why telegram bot is cut down periodically?

Question

Python

Hello.
The bot script is written in Python 3, Flask web server.
It is on Debian hosting from Digital Ocean. There is nothing else on the server.
I run the script in the background with the command
nohup python3 bot.py > out.log &
Sometimes the bot lasts a month, sometimes a week, sometimes it crashes after two days.
What do you think could be the reason?