I can confirm the issue. It still exists in the current Ubuntu 20.04.4. Adding multiple .deb files works fine, trying to add multiple .dsc and .changes files fails.
$ lsb_release -rd Description: Ubuntu 20.04.4 LTS Release: 20.04
$ apt-show-versions | grep reprepro reprepro:amd64/focal-updates 5.3.0-1.3~ubuntu20.04 uptodate
I can confirm the issue. It still exists in the current Ubuntu 20.04.4. Adding multiple .deb files works fine, trying to add multiple .dsc and .changes files fails.
$ lsb_release -rd
Description: Ubuntu 20.04.4 LTS
Release: 20.04
$ apt-show-versions | grep reprepro amd64/focal- updates 5.3.0-1. 3~ubuntu20. 04 uptodate
reprepro: