Collection not cleared after closing Power apps canvas app

My Power App has Collection1.

I have Gallery1 too.

I use the collection to track if all gallery checkboxes are checked so I can enable a button.

AgreeCheckbox in Gallery1 has the following actions:

OnCheck : Collect(Collection1, ThisItem)
OnUncheck : Remove(Collection1, ThisItem).

And then on the displaymode property of the button:

If(CountRows(Collection1) = CountRows(Gallery1.AllItems),  DisplayMode.Edit, DisplayMode.Disabled)

I'm running the App in preview mode, but sometimes items get stuck in Collection1 and the button is enabled even if only half of the checkboxes are checked (I have 6 and 2 or 3 items get stuck).

I added OnStart: Clear(Collection1) to Screen1, but it didn't work.

What's wrong?