Possibly related.  I am unable to obtain output from cups-pdf through the mate desktop.  The pdf file is created but it is left in `/var/spool/cups/` rather than `/var/spool/cups-pdf/username/` and has a name in form `d01676-001`.

If I open the cups webpage then cups-pdf shows this:
`
cups-pdf-1672  	Unknown  	Withheld  	168k  	17  	pending since
Fri May 15 11:03:09 2020 
"Backend failed" 	
 
cups-pdf-1676  	Unknown  	Withheld  	58k  	Unknown  	pending since
Wed May 27 10:58:52 2020 ` 	
 

This behaviour began after January 20, 2020.

And cups-pdf was installed through pkg.

This is what is in /var/cache/pkg relating to cups-pdf:

find /var/cache/pkg -type f -name \*cups\* -exec ls -D '%F' -l {} \; | sort -k6 | grep cups-pdf
-rw-r--r--  1 root  wheel  23376 2018-10-25 /var/cache/pkg/cups-pdf-2.6.1_4-4e9e1e3d16.txz
-rw-r--r--  1 root  wheel  23836 2019-03-30 /var/cache/pkg/cups-pdf-2.6.1_4-23dc9778df.txz
-rw-r--r--  1 root  wheel  26360 2020-03-17 /var/cache/pkg/cups-pdf-3.0.1-40d4d99245.txz
-rw-r--r--  1 root  wheel  26376 2020-05-24 /var/cache/pkg/cups-pdf-3.0.1-152f428636.txz

The update to cups-pdf-3.0.1 on 2020-03-17 seems to be when the problem started.

And, finally, when printing from mate to the cups-pdf printer the progress window reports that the backend had a segmentation fault but I cannot find this recorded in the logs.

about cups-pdf version 3.0.0
cups-pdf crashes at code
 group=getgrnam(Conf_Grp);
    grpstat=setgid(group->gr_gid);
while setting new gid for own process. Conf_Grp is NULL.

this is because conf file assumed /etc/cups/cups-pdf.conf and cannot be opened. And at this stage nothing can be logged (previous function read_config_file(filename); tries to do log_even, but log file still not open...)

And this is because of mistake in Makefile: in pdf-conf.h now contains
/* location of the configuration file */
#define CP_CONFIG_PATH "/usr/local/etc/cups"

but current Makefile tries to correct this:
post-configure:
        @${REINPLACE_CMD} ${_SUBST_CMD} ${WRKSRC}/extra/cups-pdf.conf
        @${REINPLACE_CMD} -e \
                's,CPCONFIG "/etc/cups/cups-pdf.conf",CPCONFIG "${ETCDIR}/cups-pdf.conf",' \
                ${WRKSRC}/src/cups-pdf.h

So solution:
1) Makefile should be corrected like this:
--- Makefile.orig	2020-06-07 15:42:18.182963000 +0300
+++ Makefile	2020-06-07 16:50:19.899876000 +0300
@@ -78,7 +78,7 @@
 post-configure:
 	@${REINPLACE_CMD} ${_SUBST_CMD} ${WRKSRC}/extra/cups-pdf.conf
 	@${REINPLACE_CMD} -e \
-		's,CPCONFIG "/etc/cups/cups-pdf.conf",CPCONFIG "${ETCDIR}/cups-pdf.conf",' \
+		's,\#define CP_CONFIG_PATH "/etc/cups",\#define CP_CONFIG_PATH "${ETCDIR}",' \
 		${WRKSRC}/src/cups-pdf.h
 
 do-build:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~`

2) if your ever edit cups-pdf.conf and set Grp to nonexistent group, e.g. lp, cups-pdf will crash silently. So I propose add some code to tell about it to stderr:
--- src/cups-pdf.c.orig 2017-02-24 18:32:46.767977798 +0200
+++ src/cups-pdf.c      2020-06-07 16:18:14.701263000 +0300
@@ -374,7 +374,13 @@
   (void) umask(0077);
 
   group=getgrnam(Conf_Grp);
+  if (group != NULL)
     grpstat=setgid(group->gr_gid);
+  else {
+         fprintf(stderr,"Get group data (getgrnam) failed! group name was %s\n",Conf_Grp);
+         fprintf(stderr,"GID of process not changed!\n");
+         return 1;
+  }
 
   if (strlen(Conf_Log)) {
     if (stat(Conf_Log, &fstatus) || !S_ISDIR(fstatus.st_mode)) {
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
put this in file like patch-group_cups-pdf.c and put it inf folder files/ (create int port's folder).

(In reply to taras.filatov from comment #5)
Sorry, mistake -- it's about cups-pdf-3.0.1...

Has this already been fixed by Bug 246955?

This does appear to have been fixed quite some time ago in git commit 709dd0f0 to the ports repo which applied a patch that basically did what was suggested in comment 5, but did not reference this issue in bugzilla.

And I should note that I've been using cups-pdf after the patch since around June of 2020, I should have closed the issue myself.