theme got replaced by other theme with same name on automatic updates

More information needed.

What was the name of the theme? Where did it come from? Which theme replaced it?

It was called Chicago.

I bought it here:
https://themeforest.net/item/chicago-restaurant-cafe-bar-and-bistro-theme/10069477

And it was installed with this (free) theme:

https://catchthemes.com/themes/chicago/

That’s strange, they’re from different companies. The two themes don’t even have the same name: one is Chicago; the other is Chicago Pro.

When I tried to download the theme package from Envato, and re-install the same thing happened. Very odd. Very odd!

I think you should contact the support for one or both of the plugins.