The erroneous tag can be seen by adding chek on mesh consistency (call to MMG5_chkmesh) at the beginning of the loadbalancing function of ParMmg (PMMG_loadBalancing) and by calling the ls-CenIn-2 test case of ParMmg:
  - we try to perform a swap23 on a tetra with 2 boundary faces. Along the face at interface with the neighbour which with we perform the swap, the edge that doesn't belong to a boundary face has a wrong tag in the xtetra (due to a previous collapse that has added the xtetra). In the neighbour, the edge has the suitable tag. Leading to inconsistency.

…mg-edge-tag-consistency