zypper dup does do a downgrade instead of doing an upgrade, e.g.:
Die folgenden 6 Pakete werden den Anbieter ändern:
  perl-common-sense   3.74-bp154.1.49 -> 3.5-150500.4.2.1       x86_64 ->
noarch  OSS-15.5-OBS_Server_2.10  openSUSE -> obs://build.opensuse.org/OBS
  perl-File-Sync      0.11-bp154.1.21 -> 0.11-150500.1.1        x86_64         
  OSS-15.5-OBS_Server_2.10  openSUSE -> obs://build.opensuse.org/OBS
  perl-GD             2.73-bp154.1.84 -> 2.51-150500.2.2.1      x86_64         
  OSS-15.5-OBS_Server_2.10  openSUSE -> obs://build.opensuse.org/OBS
  perl-JSON-XS        3.04-bp154.1.18 -> 2.34-150500.8.2.1      x86_64         
  OSS-15.5-OBS_Server_2.10  openSUSE -> obs://build.opensuse.org/OBS
  perl-Socket-MsgHdr  0.05-bp154.1.25 -> 0.04-150500.20.2.1     x86_64         
  OSS-15.5-OBS_Server_2.10  openSUSE -> obs://build.opensuse.org/OBS

zypper does here a downgrade for
perl-common-sense   3.74-bp154.1.49 -> 3.5-150500.4.2.1
instead of doing:
perl-common-sense   3.74-bp154.1.49 -> 3.75-bp155.1.5

compare above downgrades with following available packages:
S | Name               | Type  | Version         | Arch   | Repository
--+--------------------+-------+-----------------+--------+-----------------------
  | perl-common-sense  | Paket | 3.75-bp155.1.5  | x86_64 |
openSUSE-Leap-15.5-Oss
  | perl-File-Sync     | Paket | 0.11-bp155.2.10 | x86_64 |
openSUSE-Leap-15.5-Oss
  | perl-GD            | Paket | 2.76-bp155.1.7  | x86_64 |
openSUSE-Leap-15.5-Oss
  | perl-JSON-XS       | Paket | 4.03-bp155.1.5  | x86_64 |
openSUSE-Leap-15.5-Oss
  | perl-Socket-MsgHdr | Paket | 0.05-bp155.2.11 | x86_64 |
openSUSE-Leap-15.5-Oss

so what is wrong here with zypper ?