Does the Power Apps button that triggers the flow do anything else, such as resetting the form? Could this be a race condition where the form is reset (and the first item of each drop down is selected) before the Flow gets called?
Although it was working fine before July 27, MS do change the behaviour of the platform regularly which can result in bugs and different behaviours within existing Apps.
Try using Monitor to see what is happening in what order in your App. If the App behaves differently in Studio compared to normal running, you can use Monitor to check what is happening in the published App (but only on Web, this doesn't work for mobile) by clicking the ... next to the App in the App list and choosing Monitor then once Monitor opens you can click 'Play Published App'.
Another thing to check is whether the App was republished on/about July 27. If so, this change in behaviour may be the result of publishing on a newer Power Apps release. You could try using Settings to set the authoring version back to an earlier version to see if that helps but it is a temporary fix as you can only choose to go back the last four or five releases, so if in a few weeks you need to make a minor change and republish you will no longer be able to select the 'working' Power Apps release.
I have quite a few Apps in use and have been surprised/shocked how often platform changes break Apps that have worked fine for years.
