.... and you are correct, HF3000. Your cruise speed lodged as Mach is converted to a TAS in our system for the level and forecast temperature at that level you plan. We can however then interact with it and put it in as Mach, ... so I don't really know why it starts off expressing it as TAS.
And,... when your B767 heads off to Perth climbing to FL380 and requests amended FL260, the TAS field stays the same unless we manually change it. TAAATS calculations for entry/exit to lateral conflict points and time-of-passing etc might possibly be in considerable error unless that TAS field on the FDR is corrected for the new level