To reproduced the issue I've created a job which have 2 StringPropertyValues as job parameter:
parameter 1:
name: key
value: default value

parameter 2:
name: key2
value: default value

and a simple batch script which dumps the values:
echo key: %key%
echo key2: %key2%

if I call "!build myjob now" then I got:
key: default value
key2: default value

but if I call "!build myjob now key=myvalue" then I got:
key: myvalue
key2:

I am expecting to have:
key: myvalue
key2: default value

I've made a pull request with the relevant fix to handle properties default
value correctly:
https://github.com/jenkinsci/instant-messaging-plugin/pull/10