This is somewhere between een bug report and a feature request (since I do not no whether the not so useful behavior is intendet like it is):
The shortcut <destination> - <move modwheel> only works once. If there is already one modwheel entry in the mod-matrix, the shortcut does not do anything anymore. It would be natural to have it either 1) create a new mod slot for modwheel (the same behavior as when using the shortcut the first time; this would be my preferred behavior), or 2) jump to the existing modwheel mod slot.