[Sam James]

Currently the Panasonic 50/60P AVCHD format is nearly impossible to be edited. Even with the latest update of Final Cut Pro X,we have not seen the expected support for 50/60p AVCHD MTS. To achieve your goal smoothly, you can transcode them to FCP native format, say, Apple ProRes encoded mov format along with some help from a 3rd-party software (e.g. Pavtube MTS/M2TS Converter for Mac). Below is simple guide for you to take as reference:

https://www.pavtube.com/guide/import-edit-50p-60p-avchd-in-final-cut-pro-x-on-mac.html