The recent King Air 350i from MilVis encoded their aircraft.cfg as Windows CrLf UTF-8 with BOM.
The WIndows API will see this file with the first character missing and miss out the first section of the file [fltsim.0] and go on to [fltsim.1].
Since Ideal Flight also uses the Windows API to read those files the first section is missing and the file read aborted. That shows as a blank line in IF.
All we need to do is add an empty line at the top of the file and the King AIr 350i will become available. The aircraft may change to the more usual P3D format of UTF-16LE and UTF-8.
It is possible any type of addon may get the file mis-read. In the meantime the reliability of these aircraft will improve with a blank line at the top of the aircraft.cfg and any others in the folders with the same format and section format text. XML files are unaffected
Ideal Flight will in future read these files another way.