What is invalid internal_flags (0x28000) warning?

I have an external hard drive with 1 HFS+ partition and 3 APFS partitions. When I run Disk First Aid (in High Sierra), it finds no problems with the HFS+ partition. It also reports "drive appears to be OK" for the APFS partitions, HOWEVER I get several warnings like this on each of the APFS partitions:

warning: inode_val: object (oid 0xHHHHHH): invalid internal_flags (0x28000)

where 'HHHHHH' is a hex number.

I've searched online and can't find any mention of a flag value of 0x28000. What does this mean? Is it anything to worry about? Please explain.