Yeah I fully agree. It was a surprise to me when I first used (and sold it…) that prefixes were not available. It was just assumed like you say prefixes are a standard option of order tags that have been around since V4 and I know many people use them.
One of my biggest issues with the android app is the workarounds you need to do like this. It would appear “free tagging” has been highlighted as an important feature of the app, however the way I always see free tagging is a last resort - if you use inventory, it messes it all up when staff use that, and if you have order tags with prices (we do for some e.g. with “extra” prefix), free tagging would be a means to get around that, either accidental or deliberate.
We used to have to use QMX instead of the android app as it had support for prefixes from the start and have a few customers happy with it, but unfortunately QMX is no longer an option.