When my plan expired 3 weeks ago, I exported all my chats, then went into Data controls > Delete all chats and clicked the button "Delete all".

It behaved odd, most messages were no longer accessible, but still in the sidebar. after some time, they were all readable again.

In short: Their "Delete all chats"-feature is broken.

I just hope that they don't mine my chats in order to use the data for advertising.

might just be a caching issue or deletion taking a while, especially if you have a lot of chats