Oops I think I injected into muscle - panicking

Answering the narrow version, because the broad one does not have a single answer. There is also a quality change that the scan cannot see. Weight loss shifts fiber-type distribution toward Type I and improves mitochondrial density and insulin-stimulated glucose uptake per fiber — so the muscle you keep is metabolically better even though there is less of it. That does not make the loss irrelevant, but it explains why function often holds up better than the number suggests.

Dr.RaviCardio said:

There is also a quality change that the scan cannot see.

Agreed on the ranges, with the condition that a single DEXA is not a measurement, it is one draw from a distribution. Hydration status moves the lean compartment by a kilogram or more between scans, so any comparison of scans done at different times of day or different hydration states is noise.

If somebody has the primary source to hand I would rather cite it than paraphrase it.

nick_newbie said:

I lifted three times a week throughout and still lost lean mass.

Same position here, arrived at the long way round. The intervention that works is boring and well evidenced: resistance training two to three times a week at a genuine effort, protein at 1.4 to 2.0 g/kg of reference body weight, and not making the deficit larger than the training can support. Cardio does not protect lean mass and in a deep deficit competes with recovery.

From the other side of the consultation, briefly.

OMG yes. lean mass + resistance training is the combo. Can confirm.