How to disable the new "rename file" behavior in Windows 10 (hitting F2 and cursor left)?
Windows Explorer has become much more user-"friendly", and does a very good effort to protect you against yourself.
To this end it pre-selects the file-name part, while excluding the extension part, so you will not by mistake change the extension and make the file unusable by any application that registered that extension.
Unfortunately, pressing Left-arrow while the text is selected causes the cursor to be placed to the left of the selection, which is at the beginning of the file-name.
The solution is simple: Use instead the Right-arrow key. This will place the cursor to the right of the selection, which is exactly where you want it to be.