no, I don’t have any other idea. What you could try is to keep only one codec in the AMR FR codec set, ie. only the 12.2kbps.
Check if it is better.
Then add a second codec, for exampe, 10.x kbps. Check the FER.
a) If NOK -> Remove the codec 12.2 and use only 10.x -> check again.
b) If OK -> Then change this second codec to a lower one, and check the FER again. At this stage, you use only 2 codecs in the set.
Then once all codecs are verified, you should start the same thing with 3 codecs.
By the way, using only these 2 codecs : 12.2 and 7.4 is probably what I would apply on the field.
You don’t really need 3 codecs, except if you want to spend a lot of time optimizing your C/I threshold for very little benefit.
12.2 : good VQ
7.4: good noise protection + TFO compatibility with AMR HR (in B11 only)