Check this - Brother “Scan to PC” button not working across VLANs/separate networks on RB5009 — mDNS RouterOS 7.20.8 - #9 by optio
Not sure you have same issue, but I had issue that input packets was dropped from VLAN to dst 224.0.0.251:5353 udp. I added rule as sugested (with adjusted to my case where in interface is set to VLAN) in that topic and issue was resolved.