According to Twitter user Devin Meredith, “Apple just reached out to me to see if I could answer some questions about my XS Max reception issues. They’re doing an outreach to some affected users and are asking to install a baseband logger to track my connection to the tower.” Note that Apple doesn’t seem to have a fix at the moment and the request to users to install a logger suggests that they are still investigating the problem.
This means that the problem could be just about anything, ranging from hardware to software or maybe even carrier firmware. AppleInsider has confirmed with their sources inside Apple that there is a “limited data collection effort” currently in progress, which reaffirms what some users have been reporting as well.
No word on when a fix could be released, but last we heard iOS 12.1 will contain fixes for the charging issues in the iPhone Xs, so maybe a later build of iOS 12.1 could contain a fix for the LTE issue, assuming that it is software related.