There are no updates to our application that would explain the issue. Our tests are backwards compatible with previous versions of our app. So, I tried it in an older version. I can reproduce the problem with a version that was released last year and has not been updated/rebuilt since then.
It's happening for all buttons in dialog boxes and toolbars. Hence the mapping properties vary. In one case the button is mapped using WndClass and WndCaption. The "Add" button in my example is mapped with ObjectType and Caption.
I believe the problem started occurring after a recent TC upgrade.
If it's after upgrading to the latest version of TC, then I suggest you open a ticket via https://support.smartbear.com/testcomplete/message/
