As far as I know, you can print multiple files with a single lpr command, and if I issue a command like this

lpr fileA.txt fileB.txt

both fileA.txt and fileB.txt are printed. But if I write:

lpr fileA.pdf fileB.pdf

only fileB.pdf is printed.

What do I do wrong?

I'm running Ubuntu 20.04.1 LTS.

EDIT

After printing, /var/log/cups/access_log contains this:

localhost - - [17/Dec/2020:19:39:19 +0100] "POST /printers/HP_OfficeJet_Pro_6970_7BE7A4_ HTTP/1.1" 200 490 Create-Job successful-ok

localhost - - [17/Dec/2020:19:39:19 +0100] "POST /printers/HP_OfficeJet_Pro_6970_7BE7A4_ HTTP/1.1" 200 6990 Send-Document successful-ok

localhost - - [17/Dec/2020:19:39:19 +0100] "POST /printers/HP_OfficeJet_Pro_6970_7BE7A4_ HTTP/1.1" 200 7085 Send-Document successful-ok

localhost - root [17/Dec/2020:19:39:20 +0100] "POST /admin/ HTTP/1.1" 200 306 CUPS-Add-Modify-Printer successful-ok

localhost - root [17/Dec/2020:19:39:21 +0100] "POST /admin/ HTTP/1.1" 200 306 CUPS-Add-Modify-Printer successful-ok

And /var/log/cups/error_log contains this:

E [17/Dec/2020:19:39:20 +0100] [Job 261] File '' not found