Volume changes can come as quite an inconvenience to say the least! There have been a few instances of this reported over the years, but it doesn't seem like everyone is experiencing it. Though some small level of volume difference can be normal between networks, if it's more of an extreme adjustment I would definitely check to see if your TV has an automatic volume setting sometimes referred to as "Advanced Auto Volume."
Has anyone else in the community experienced this issue? What were your experiences, were you able to resolve?