Thank you for your continued help in this!
Split on was already set to on so we are good there, but I'll change the Concurrency control to the following:
Hopefully this resolves the deadlocking issue. Given what you said about Power Automate not allowing multiple edits to the same Excel file at the same time, perhaps transitioning to Power Apps may be the long-term solution (or SharePoint lists and forms).
So in summary, it may not be the Condition action or the Switch action that are causing issues and data loss, but simply the fact that I have multiple people submitting the same form that then uses Power Automate to sort that data into the appropriate tables in the same Excel spreadsheet because Power Automate won't allow multiple edits at the same time? If this is the root of the issue, changing the Concurrency control to the above settings should solve then issue, hopefully?