"Unlimited" match against AI was terminated

Hello! First, please forgive my English as I'm using Lichess in another language and I'm not sure about the terminology used in the English app.
I've just started playing chess and using Lichess, and something strange happened yesterday. I started a match against AI from the App, specifically choosing "unlimited" time. Then, during the match, I closed the app. Now I've reopened the app, and the match is terminated. I've tried starting a new one and closing the app right after, but this doesn't terminate the match.
So, what happened? Is there an "invisible" time limit even when time is set to "unlimited"?
Many thanks!

From previous experience (this may have changed since) such unlimited games against the AI are aborted after a day of inactivity. Not sure why that is, probably to free server resources or something. Not sure one can do anything about it.

@MoistvonLipwig "Not sure one can do anything about it." Not describing something which expires in a day as "unlimited" would surely go some way towards dispelling the confusion. Another solution would be for users to have access to a PGN of all games (for a limited period of there's a space problem) so that they could import the abandoned ones and continue if desired.

@Doofenshmirtz why are you telling me that? I'm not a dev, I was obviously talking about what an average user like me and you can do about it. It is not in my power to change what some game is described as.

As for the second suggestion, doesn't that just work as is? It's a bit cumbersome to import after every day though.