Yes, that's definitly a bug. There no info about port not supporting L3 mode. What's more, after "switching" to L3 mode, there are new commands avaliable, for instance some vrrp related commands. Please find an example below.
[Quidway-GigabitEthernet0/0/1]
portswitch
Error: The interface is already a L2 interface.
[Quidway-GigabitEthernet0/0/1]?
gigabitethernet-l2 interface view commands:
_hidecmd
am Port isolate
arp Specify ARP configuration information
arp-limit Limit the number of learnt ARP
arp-miss Specify ARP MISS configuration information
authentication Authentication
auto Auto negotiates port mode
---- More ---
[Quidway-GigabitEthernet0/0/1]undo portswitch
[Quidway-GigabitEthernet0/0/1]?
gigabitethernet interface view commands:
_hidecmd
admin-vrrp Specify configuration information of the
administrator VRRP
admin-vrrp6 Specify configuration information of the
administrator VRRP6
am Port isolate
arp Specify ARP configuration information
arp-limit Limit the number of learnt ARP
arp-miss Specify ARP MISS configuration information
---- More ---
[Quidway-GigabitEthernet0/0/1]
Regards