I'm sorry in advance if this is covered in another post, I did my best to see if I could find it addressed anywhere else! And thanks for any time you take looking at my problem, I really appreciate it.
I have a Canvas app based on 2 SharePoint lists - the first is a list of schools with basic information about them, the second is a list of visits to those schools. My end goal is for colleagues to be able to find the school they're interested in on the first screen of the app, which can take them to a second screen that shows them just the visits for that school. They can then navigate back to the first screen to check other schools.
This is the first screen (School screen) for my app, which works correctly.

If I click on the 'Visits' button on the first screen, it takes me to the second screen (Visits screen), displaying only the visit for Archbishop Runcie school, which is correct, along with the school's name suffixed by " Visits". So far, so good.

Next I navigate back to the Schools screen by selecting the back navigation button on the top left corner and select another school:

But when I select 'Visits' for this school, I get a 'No item to display' message on the cards, even though it's correctly showing a visit on the left gallery. It also excludes the school's name on the header.

I'm sure it's something to do with the gallery selection, since if I click 'Visits' on another school on the School Screen first, it will display their visit(s) correctly as above, and then Archbishop Runcie's visits won't display properly.
Thanks for your help, any advice massively appreciated.
Andy