Just as a followup, I have been unable to solve this issue, and it happens with two different 2100’s.
So the workaround for now is to use mvneta0 as the uplink to the switch carrying multiple VLANs in a 802.1q trunk (leaves the builtin 5 port switch out of the inter VALN routing).
At some point I will try with another managed switch model to see if it’s the specific Switch/SG-2100 combination only, or a general issue.