This is not a bug. From the Blocks Guide:
Cab 1, 2, 3, 4 Level – (two on FM3) These parameters may seem straightforward, but can actually be a bit tricky until you understand how they work. Rather than simply controlling the overall level of each Cab IR, this sets relative levels between the IRs when more than one is loaded. The final blend is then normalized and controlled via the Level parameter on the Mix page—a master control for all four Cab IRs. So, for example:
When only ONE Cab IR is in use and the other three are muted, this control will have no effect since relative level doesn’t apply without two or more Cab IRs to compare.
When TWO (or more) Cab IRs are loaded, you only need to adjust the one(s) you want quieter. Think about it: reducing Cab 1 to –2dB and Cab 2 to –4 dB is the same as leaving Cab 1 alone and setting Cab 2 to –2 dB. Only the difference counts!
With more than one Cab IR in use, you will hear the tonal blend change, but the overall volume will remain relatively the same because again, the blend is normalized as you adjust. Do keep in mind however that because of their frequency contents, some cabs may be perceived as louder or quieter than others.
