Goedjn wrote:

Collapsed pipe an/or a root-clog, further down the line,
so that there's enough volume in the pipe to take one
flush (or maybe two), but which fills up when you put
a tubfull of water into it. That will seep down over
several hours of non-use, and then appear to work again.

I second this assessment. I hope we're wrong. If we're right, break out
the wallet.