Turns out this is a server side limitation in kodi+LibreELEC:
LibreELEC # cat /storage/.kodi/userdata/upnpserver.xml
<upnpserver>
<UUID>e6ed9e0b-0580-e62c-4fc0-3a7e789f7575</UUID>
<Port>1973</Port>
<MaxReturnedItems>200</MaxReturnedItems>
<UUIDRenderer></UUIDRenderer>
<PortRenderer>0</PortRenderer>
</upnpserver>
Simply changing 200 into 400 fixed the symptoms.