I couldn’t replicate it, but I’m wondering if this simple shortcut has the same issue. Also, are your two Macs on the same macOS version?
The closest I got was that .md is ignored in the file save sub path if it’s VariableName.md, and macOS will append that with .txt If it’s all just literal strings, e.g., texting.md, it’ll save as markdown without the rename.
I’m wondering if the Macbook Air is on a version that has that same issue, but in the Rename File action.
Thanks for replying and you may be right re versions - the Air is on the Beta of Seqouia. However, if I insert a variable in the options of the save action, it all seems to work…