I just found out that I cannot edit any already entered post addresses. They show up in the list alright, but it I select the entry in the drop down list, nothing is shown to be edited.
STEPS TO REPRODUCE
1. Open any contact with a post address
2. Select the entry from the drop down list (in my case, most addresses are labeled as "private", as they are from a google address book).
The entry can only be deleted, but not edited
I should be (easily!) able to edit/change any post address
Operating System: KDE neon 5.17
KDE Plasma Version: 5.17.2
KDE Frameworks Version: 5.63.0
Qt Version: 5.13.1
Kernel Version: 4.15.0-70-generic
OS Type: 64-bit
Processors: 4 × Intel® Core™ i5-7200U CPU @ 2.50GHz
Memory: 7,7 GiB