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Description of problem:
hostnamectl set-hostname fail to set a new hostname has 63 characters
While it works with 62 or 64 characters new hostname

63 characters fail
# hostnamectl set-hostname CUSTVM-89012345678901234567890123456789012345678901234567890123.vmware.com
Could not set property: Invalid static hostname 'custvm-89012345678901234567890123456789012345678901234567890123.'

64 characters pass
# hostnamectl set-hostname CUSTVM-890123456789012345678901234567890123456789012345678901234.vmware.com

62 characters pass
# hostnamectl set-hostname CUSTVM-8901234567890123456789012345678901234567890123456789012.vmware.com

Version-Release number of selected component (if applicable):
# cat /etc/os-release 
NAME="Red Hat Enterprise Linux Server"
VERSION="7.6 (Maipo)"
ID="rhel"
ID_LIKE="fedora"
VARIANT="Server"
VARIANT_ID="server"
VERSION_ID="7.6"
PRETTY_NAME="Red Hat Enterprise Linux Server 7.6 Beta (Maipo)"
ANSI_COLOR="0;31"
CPE_NAME="cpe:/o:redhat:enterprise_linux:7.6:beta:server"
HOME_URL="https://www.redhat.com/"
BUG_REPORT_URL="https://bugzilla.redhat.com/"

REDHAT_BUGZILLA_PRODUCT="Red Hat Enterprise Linux 7"
REDHAT_BUGZILLA_PRODUCT_VERSION=7.6
REDHAT_SUPPORT_PRODUCT="Red Hat Enterprise Linux"
REDHAT_SUPPORT_PRODUCT_VERSION="7.6 Beta"

This is caused by hostname shortening AFTER cleanup, which results in a trailing dot in the static hostname in specific cases, thus resulting in an invalid hostname. This should be fixed by this patch[0], which does the shortening BEFORE the cleanup.

[0] https://github.com/systemd/systemd/commit/46e1a2278116e2f5067c35127ccbd8589335f734

Thanks Frantisek for the quick response.
Will RHEL7.6 update systemd version which contains this fix?

It's too late for RHEL 7.6, so it'll have to wait for RHEL 7.7.

https://github.com/lnykryn/systemd-rhel/pull/232

fix merged to staging branch -> https://github.com/lnykryn/systemd-rhel/pull/232 -> post

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHSA-2019:2091