The 12.1 dialog was a heavily customized one supporting an old Windows common dialog behavior.
The current behavior in 12.11 and 12.12 matches the current Windows common dialog, which no longer supports the old behavior.
As a workaround, please type i.e. *proc* and press Enter. The file list (above) will be filtered using a partial match, so all filenames that has "proc" in its name will be displayed. All of the other standard Windows path wildcards should work as well. For example, if you type proc*, all filenames that start with "proc" will be displayed. Or if you type *proc, all filenames that end with "proc" will be displayed.