pkgng+portmaster: portmaster erased the field packages.time

[Yuri]

After I converted to pkgng, field packages.time contained the time of 
conversion.

When I ran portmaster to rebuild few ports, all those timestamps 
disappeared, and portmaster only left the timestamps on the ports it 
reinstalled.
I suspect this is a bug, since this behavior doesn't appear reasonable. 
Why would portmaster erase the time field? How can it conclude it is wrong?

Yuri