  In multilayer moir\'e heterostructures, the interference of multiple twist
angles ubiquitously leads to tunable ultra-long-wavelength patterns known as
supermoir\'e lattices. However, their impact on the system's many-body
electronic phase diagram remains largely unexplored. We present local
compressibility measurements revealing numerous incompressible states resulting
from supermoir\'e-lattice-scale isospin symmetry breaking driven by strong
interactions. By using the supermoir\'e lattice occupancy as a probe of isospin
symmetry, we observe an unexpected doubling of the miniband filling near
$\nu=-2$, possibly indicating a hidden phase transition or normal-state pairing
proximal to the superconducting phase. Our work establishes supermoir\'e
lattices as a tunable parameter for designing novel quantum phases and an
effective tool for unraveling correlated phenomena in moir\'e materials.
