Edit:
I guess that's the reason :-D :
The sign is another prefab. You couldn't see it because you inserted the line
<decoration type="model" name="CraneA16_2Q" position="500, 44, 1215" rotation="0" />
at the end of the file, right?
Probably you cannot place two prefabs at the same location even if 'copy air blocks' is on. Remove the sign in the prefab.xml or try this workaround:
https://7daystodie.com/forums/showthread.php?62501-Prefab-Editor-(Alpha)&p=722270&viewfull=1#post722270
So make an extra air prefab and add this thing to the prefab.xml between sign and CraneA16_2Q:
...
<decoration type="model" name="sign????" position="?, ?, ?" rotation="0" />
...
<decoration type="model" name="Air" position="500, 44, 1215" rotation="0" />
<decoration type="model" name="CraneA16_2Q" position="500, 44, 1215" rotation="0" />
...