Heroku is throwing different data on the page while the server is running, why is that?

Question

Flask

Hello. We are making a training project of a messenger on flask. deploy to heroku. Everything is still working, but here's what strangeness they noticed. For starters and simplicity, the database was made simply as a list in a server file. There is a page that displays this list. And now, if this page is updated, the data from the list either disappears (the list becomes empty), then they appear again! I already understood that heroku does not store data like text files, etc. but after all, the list is part of the python program, if the server does not fall asleep, where does the data go from it, and most importantly, how do they reappear later?! Moreover, you can refresh the page, say 10 times in 30 seconds, and the output will change several times! Can this affect the logic of the server, or does it only affect the output on the page?
I forgot to add, the same server on LAN does not have such a bug, i.e. the problem is obviously on the heroku side?..