Is it a bug?
I have custom properties in my library feature parts that transfer just fine. Are there configuration specific and non-configuration specific properties in the file with the same property name? In that case I believe the configuration specific property would override the other one. If this doesn't answer your question, can you download the library feature part file?
Very strange but my test profile for you works well now on SW2018 sp0. Tomorrow I`ll check it on sp3. I`m sorry to worry you.
New test file works well on SW 2018.3. So, profile file which doesn`t transfer common properties is attached.
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