Hey Karel, Shaq from the Support Engineering team here!
Is your Checklist step linked to a Product Tour? If so, it would be expected behavior for the Checklist to close during a Product Tour. The tour will show and the Checklist will hide. The Checklist will also not reopen automatically once the tour is completed.
The Checklist's behavior is designed to show progress and encourage users to take the next step by reopening to notify them of their progress when a step is automatically resolved using rules you create. To reopen the messenger, you could either can make an API call or you’re correct, customers would have to reopen manually by clicking on the widget.
Outside of a Product Tour, the Messenger and Checklist should reopen when navigating to a new tab from a Checklist, as long as the messenger is installed on the new page. So if your Checklist is not tied to a Product Tour, it sounds like this may not be working properly. I went ahead and created ticket with our Support team on your behalf so we can take a closer look at this for you.
Thank you!