BW> I just tried fetching the "new" robots.txt via Mystic's FTP server, but
 BW> it appears to give me the QWK file instead:
 BW> 
 BW> + 2022.05.05 21:08:06  FTP 1-C: RETR Data: SCBBS.qwk

The log is showing that your FTP client is requesting the QWK packet for some reason.

When I try it here with FileZilla it seems to work but I do seem some oddities at the end where it looks like it gets some sort of blank command from FileZilla.  Here is what my log looks like this:

+ 2022.05.05 17:07:14  FTP 1-C: CWD Data: /
+ 2022.05.05 17:07:14  FTP 1-S: 250 Working directory is now "/"
+ 2022.05.05 17:07:14  FTP 1-Changed to /
+ 2022.05.05 17:07:14  FTP 1-C: TYPE Data: A
+ 2022.05.05 17:07:14  FTP 1-S: 200 All files sent in BINARY mode.
+ 2022.05.05 17:07:14  FTP 1-C: EPSV Data: 
+ 2022.05.05 17:07:14  FTP 1-S: 229 Entering Extended Passive Mode (|||60000|)
+ 2022.05.05 17:07:14  FTP 1-C: RETR Data: robots.txt
+ 2022.05.05 17:07:14  FTP 1-S: 125 Data connection already open
+ 2022.05.05 17:07:14  FTP 1-Sending: \mystic\data\robots.txt
+ 2022.05.05 17:07:14  FTP 1-Send complete
+ 2022.05.05 17:07:14  FTP 1-S: 226 Transfer OK
+ 2022.05.05 17:07:27  FTP 1-C:  Data: 
+ 2022.05.05 17:07:27  FTP 1-S: 502 Command not implemented.
+ 2022.05.05 17:07:27  FTP 1-Connection closed

... Top secret! Burn before reading!

--- Mystic BBS v1.12 A48 2022/04/26 (Windows/64)