Can't Import RF64 Multichannel Poly WAV anymore?

Has something changed in FCP 10.5 with multichannel surround importing? It no longer works for me in 10.5.1. FCP supports RF64 files. I had no issues importing 5.1 (6 channel) polyphonic files in 10.4.9.  I am using FCP 10.5.1 on Big Sur and cannot import 5.1 stems and the print master from ProTools.  I've never had this problem before.  They import fine in Da Vinci Resolve and Adobe Premiere Pro. Also interesting is the files show no duration in the import window, however they work in other NLEs and are about 4.5 GB in size each. The only workaround I've found so far is opening the files in Logic Pro and re-exorting them as AIF files. Then they import into FCP.  What's changed?