Hi,
Than you for the report.
UPnP renderers volume range is usually in the 0-100 (although it can technically be anything), although internally they might use a different range.
BubbleUPnP does not change the volume (here to 100) on its own.
This 100 value is communicated by the Neutron renderer to BubbleUPnP, hinting at a bug in Neutron if its actual volume value is not 100 (whatever internal volume value it uses, mapped to the UPnP 0-100 range).
In BubbleUPnP, there is no way to limit the max volume (say to 30 or 50) but the app do its best to prevent accidental volume scares. But that only works if the renderer
reports to the app its current volume properly, which is not the case here.
I will see I can reproduce this issue with the Neutron renderer to eventually report it to its developers.