I too have experienced the import glitches when importing into FCPX from a Canon Vixia HF G10 camcorder. The ClipWrap workaround is effective as it can open multiple .mts files as “Spanned” and convert into a single .mov file suitable for Final Cut Pro X and does not exhibit the glitches.
Unfortunately, this import anomoly continues with Final Cut Pro versions 10.4.1 and as of yesterday, 10.4.2. I sincerely hope Apple addresses this bug in the next version release.