I guess you accidentally double-clicked your pool node. This currently triggers the "View Group Content" action (see "Grouping" menu) - which is an unintended side effect of a bug fix in yEd 3.17. If you used "Undo" for reverting back to the previous state instead of "Grouping" -> "Previous View", this may have left your pool/swimlane node in the "closed" state. Currently, the only way to open the pool/swimlane node once again is using "Undo" once more.