Set up a text filter, freetext, with a default value. Save it. Now, edit the filter, change it to a node property filter and choose your node and property. The default value will still show - don't change it. Save the filter. The default property value will now have disappeared, even one that is valid. Editing the filter again allows you to re-set the default property value, and this time it will stick.

If you edit the default value at the point where I said "don't change it", then this issue doesn't occur.