  In 3D topological insulators, an effective closure of the bulk energy gap
with increasing magnetic field expected at a critical point can yield a band
crossing at a gapless Dirac node. Using high-field magnetooptical Landau level
spectroscopy on the topological crystalline insulator Pb1-xSnxSe, we
demonstrate that such a gap closure does not occur, and an avoided crossing is
observed as the magnetic field is swept through the critical field. We
attribute this anticrossing to orbital parity and spin mixing of the N=0
levels. Concurrently, we observe no gap closure at the topological phase
transition versus temperature suggesting that the anticrossing is a generic
property of topological phase transitions.
