I also own Asrock C2550D4i
I've never tried anything prior to 5.1, but from my last known experience, 5.1 also has "=trigger device plug event=" issue, I heard from some people in Chinese forum that 5.0 or earlier version has no such issue.
As long as I boot the machine with 9172 enabled, even with no HDD connected, the mentioned problem comes up, but this issue will be gone if I install full feature Linux/BSD (I tried with OpenMediaVault, which is based on Debian Linux, and FreeNAS/NAS4Free which are based on FreeBSD 9/10), I'm pretty sure that this is DSM kernel specific.
Looking back to previous post, I think I will try to disable IOMMU (either in BIOS or boot prompt) to see if that can help.