In addition to what I wrote in my previous post, you’ll want to set "opt_coordinates": "both".
If you know what to look for in the output file, you can tell that the geometry optimizer is having some difficulty converting between displacements in internal coordinates and displacements in Cartesian space. The command I posted tells the optimizer to treat Cartesians and internals on a more equal footing when calculating displacement. This is slower, but lets the system converge.
Paging @Rollin_King. This should be on your radar for the geometry optimizer rewrite, if it hasn’t been already.