This seems to have started with the changes to finder in Monterey.
When saving a pdf from illustrator, I routinely save a copy to a folder called "pdf". The location of the folder is different for most files (based on the job). When the "save as" dialog pops up to navigate to the location of the file, when I click on the folder named "pdf", the filename field changes the name of the file to "pdf.pdf". Basically its inheriting the name of the folder you click on first when navigating through the save dialog, so if the folder is named "stuff", the file name will change to "stuff.pdf".
I've gotten to the point now where the first thing I do is highlight the file name and copy it in case when I get to the destination folder it changes the name, I can just paste the correct name back into the filename field.
I haven't noticed this in any other applications, but it's obviously due to all the things Apple changed about finder in the latest update.