I haven't worked with those files directly inside FCPX for a while - or better said: never.
For good reasons I didn't touch them since FCP 5 or so.
One of the reasons had been TC. When Apple told me that they finally have integrated BWF reading they implemented a stupid error when reading BWav TC. I told them, they agreed but never fixed.
Now I got a user question and have to care about this again after more than a decade.
File header shows a time of "samplesSinceMidnightLow" of 4.090608E+9 at 48kHz
Simple math results in a time of 23:40:21.000 whereas FCPx says: 23:38:55.833333333.
Other apps follow the "simple math" as well.
fcpxml doesn't have a "virtual tc" like xmeml had. It also doesn't accept the TC overwrite from iXML.
So what to do?
Regarding iXML and "Assign iXML track names if available":
When a stereo track has a name for each track FCPx only assign one.
Does all the tracks need to be mono?
Partially answering my own question:
The example file mentioned did have an iXML. This included a SPEED: 24 and a TIMECODE_FLAG: DF which results in frame rate of 23.976
The above mentioned time of 23:40:21.000 equals 85221 seconds - latter are displayed correctly in FCPx.
Now with a given video rate FCPx will convert the audio into "video frames" and the time (timecode) is converted to 23:38:55:20.
This means going from absolute time based to relative frame based.
Whether this is right or wrong can be discussed - in any case it is problematic.
Having several audio sources with time stamps will not sync or run out of sync in case they have an iXML or not.