So you have 2 issues, right ?
1/ the weird IMSI, that we don’t see in France (all IMSI are 15 digit, and use correct country codes)
2/ ??? related to weird PLMN ID, which is caused by Qualcomm specific implementation for US operators? This PLMN id are found in the EPS mobile id/oldGuti of the weird IMSI, yes? And nowhere else?
Don’t you think that due to the misformat of PLMN ID, the whole hexa message is offset and misdecoded by your trace tool ?