PyCharm Debugger hides variable values by default

The PyCharm debugger doesn't show a preview of variable values anymore. Only the variable name is shown, with a button "Show Value" on its right:

Simple strings or integers don't seem to be affected. Other types (tuple, counter, dict, list) and their content however are hideen by default.

Clicking on every single variable in order to inspect its value is really cumbersome. As far as I know, I haven't changed any settings. I already tried invalidating caches and restarting, and the problem stays across projects. How can I get the normal variable preview back?