I referenced the wiki which is generally the source of truth.
As @jow wrote in 2020:
Suffix only works for clients doing stateful DHCPv6, it has no effect on systems not doing DHCP but SLAAC only, e.g. Android
The hostid suffix value is limited to 32bit, so the maximum possible value is ffffffff
That's because scanning the number stops at the first non-digit, so only be51 gets used in this case.
That value is too large to fit into a 32bit integer. be5107fa or f82238d7 would work.
32 bits = 4 bytes = 8 (hex) characters FF-FF-FF-FF.
If you @openwrtforever have experimental evidence to the contrary, I would be interested to see it. ( Things may have changed internally since @jow wrote the above).