Hello, I have discovered an issue with the new Save As and first time Save window. The GUI implies that files are being saved as ".pbix". However, if you enter a file name that has a "." in it, the file will be saved as a file type of whatever is after the ".". For example, I have saved a file called "Report v1.12" and it has saved the file as a file type of "12". The user can resolve this by adding ".pbix" to their file name, but the GUI is misleading the user into thinking that ".pbix" is being added automatically --- surely this is a simple fix!
I did reproduce the problem as you did, and unlike the September version, it provides a more convenient way to do it, which may be by design, just as filename expandability can be hidden in computers. You can avoid this problem by using "Browse this device".