This has been the behaviour for every mIRC version I've tested, and I'm not sure if there's an intentional reason for it. To reproduce:
1. Go to mIRC_Options/DCC/Folders and create a DCC Get folder for filetype *.txt or any other filetype that does not have a Get Folder defined for it - point it to any newly created foldername you wish. You can test this with any other filetype you wish, just substitute that .ext for .txt herein.
2. Ask someone to DCC Send any .txt file to you that is not currently located in your $mircdir folder, and the file downloads into the folder you create.
3. Now delete or rename the newly created folder, and receive that same file again.
You'll find that if a Get Folder is defined, and that foldername doesn't exist because it has been deleted or renamed, the file downloads into the mIRC program folder where mirc.exe is located. It doesn't seem to matter whether the missing Get Folder was located on a different drive letter or not, it seems to always default to the mIRC folder itself.
In my opinion, Instead of doing this, mIRC should either:
A. Use the 'default' Get Folder if it exists.
B. Refuse to accept the file until the Get Folder exists, with an appropriate message to the Status Window.
It's easy for someone to accidentally move or rename a folder, forgetting that it's an mIRC Get Folder. There are plenty of scripts out there that read+evaluate from $mircdir text files in ways that could become an exploit if someone accentally moves/renames their .txt Get Folder.