What is invalid internal_flags (0x28000) warning?
The reason you cannot find it is that there's no single flag with a value of 0x28000 - rather that is the value of internal_flags, which is a combination of multiple flags.
In particular 0x28000 corresponds to a combination of:
0x20000 = INODE_FAST_PROMOTE
0x08000 = INODE_NO_RSRC_FORK
INODE_FAST_PROMOTE means that the inode is scheduled for promotion from slow storage to fast storage. I.e. the next time the operating system reads in this inode, it is supposed to be promoted.
INODE_NO_RSRC_FORK means that the inode does not have a resource fork.
Why this combination is illegal depends on the state of your system, which you haven't specified. For example INODE_FAST_PROMOTE is only valid on a Fusion drive, and particularly it is only valid on an inode stored on the slow drive in the Fusion drive.
This particular warning could come from various causes depending on your specific system - for example:
A) You have a Fusion drive and this inode was migrated from the slow drive to the fast drive without removing the flag. It would be an operating system bug.
B) You haven't got a Fusion drive and this inode was tagged for promotion anyways. That would be an operating system bug.
C) A hardware error occured with the drive, interfaces, or other place that caused this particular bit to flip inadvertenly.
So to sum it up: If this is the only warning you have, and you experience no other problems - I would take care to ensure that you have working backups, and then disregard it.
If this is only one of many warnings, and you have experienced that more and more warnings are given - I would highly suspect that the drive is about to fail and needs replacement.
If you absolutely want to get rid of the warning, I would suggest taking a full backup - reformat the drive - and restore the backup.