How to bridge a subinterface across an HPE MSR2003 router (Comware 7)?

The setup I'm trying to achieve is G0/1.20 bridge to G0/0.20 (no routing, just bridging).

The equivalent commands on a Ubiquiti USG-3P (for reference):
set interfaces bridge br0 stp false
set interfaces ethernet eth0 vif 20 bridge-group bridge br0
set interfaces ethernet eth1 vif 20 bridge-group bridge br0

I am able to configure the main interfaces as a bridge:
[RouterHPE-GigabitEthernet0/1]port link-mode ?

bridge Switch to layer2 ethernet

route Switch to layer3 ethernet

[RouterHPE-GigabitEthernet0/1]port link-mode bridge ?

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But not on the subinterfaces:
[RouterHPE-GigabitEthernet0/1.20]port ?

link-aggregation Link aggregation group

When I set the main interfaces as a bridge, all traffic is essentially bridged, but I need routing to occur between several other VLANs.

I've tried googling but I can't seem to find any info on bridging subinterfaces on a HP router.
Thanks a lot guys