Is this new or intermittent behaviour, or have you noticed this from the beginning?

It is quite normal for volume levels to differ when you switch patches. Most people tweak their patches so that the levels are consistent when switching. It is also quite normal for levels to change dramatically when you move the Vol knob; in that case the level is jumping immediately from the stored value to the value of the physical knob position. If those values are very different the change will be instantaneous, and very noticeable.

However - if you previously had leveled the volumes between patches, and you are now noticing new behaviour, then that's an issue that needs to be resolved.