We got the Multiproperty to work well with EPiServer 7, but we have one issue. It seems the property is never initialized when the app loads or after an iisreset.
Can you post your property definition? Could it be that you have forgotten virtual keyword?
We forgot to set stringDelayedLoadThreshold="0" in episerver.config / sitesettings.
Just a side note.
If not defined in episerver.config, stringDelayedLoadThreshold is 255 by default. (in CMS 7)