Always consult the official Owners Manuals and Guides first.
Information about the FM3 may be incomplete or wrong, it's being worked on.
Spillover: when the trails of delay or reverb can still be heard after switching to another sound.
All FAS processors support spillover under certain conditions as explained in the manuals.
Spillover when switching presets
The setting in the Global menu applies to switching presets only, not to switching scenes, switching channels or bypassing blocks.
True, independent spillover is not supported because this would require double the processing power of each processor.
"Global spillover settings only apply to preset changes. Spillover between scenes is, in general, desirable and controlled by the bypass mode and state of the various blocks." source
Spillover: when switching scenes
The Global Spillover setting doesn't apply to spillover when turning off effects or switching Scenes or Channels. Delay and Reverb trails always ring out when switching scenes, depending on the effect block's Presets#Bypass_mode_of_an_effect_block Bypass Mode setting. This is inherent to the way scenes work. When it comes to spillover, scenes work better than presets and are easier to set up.
"Spillover is in there but the "right" way to do it is to use scenes. If you stay within a preset and use channels and scenes the spillover is perfect." source
Scene controllers allow more control over what happens with the sound when changing scenes.
Spillover: when bypassing effects
Spillover can be affected when switching between the X/Y states or Channels of an effect, if drastically different algorithms exist between the two states. For example, if one scene has a Digital Delay and the next scene uses a Tape Delay, spillover will probably not function correctly as these modes use different algorithms.
Forum member: "I have two patches that both has delay blocks disabled and when I switch between these patches via MFC the delay spillover sometimes kicks in really loud."
"This can happen if you rapidly switch between presets many times. The unit requires a small amount of time between preset changes to clear unused buffer space. This is done as a background task." source
If you use the Mix and Level parameters to dial in the desired effects level, the volume level of the reverb and/or delay trails may change when switching presets. This can be prevented by keeping Mix and Level at fixed values, and using Input Gain to set the desired effects level. source