  Quantum anomalous Hall (QAH) insulators transport charge without resistance
along topologically protected chiral one-dimensional edge states. Yet, in
magnetic topological insulators (MTI) to date, topological protection is far
from robust, with the zero-magnetic field QAH effect only realised at
temperatures an order of magnitude below the N\'eel temperature TN, though
small magnetic fields can stabilize QAH effect. Understanding why topological
protection breaks down is therefore essential to realising QAH effect at higher
temperatures. Here we use a scanning tunnelling microscope to directly map the
size of the exchange gap (Eg,ex) and its spatial fluctuation in the QAH
insulator 5-layer MnBi$_2$Te$_4$. We observe long-range fluctuations of Eg,ex
with values ranging between 0 (gapless) and 70 meV, uncorrelated to individual
point defects. We directly image the breakdown of topological protection,
showing that the chiral edge state, the hallmark signature of a QAH insulator,
hybridizes with extended gapless metallic regions in the bulk. Finally, we
unambiguously demonstrate that the gapless regions originate in magnetic
disorder, by demonstrating that a small magnetic field restores Eg,ex in these
regions, explaining the recovery of topological protection in magnetic fields.
Our results indicate that overcoming magnetic disorder is key to exploiting the
unique properties of QAH insulators.
