Adding the clinical framing, because it changes how the question reads. Start from the measurement rather than the conclusion. Almost every disagreement here turns out to be two people measuring different things and comparing the numbers anyway.

hank_denver said:

One practical note: write down what you did and when, before you need it.

There is a second half to this that has not been said yet. 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.

One concrete data point for the thread. The boring version of this is the one that works, and the boring version is: measure a baseline, change one variable, wait, measure again under the same conditions. Nobody wants that answer and it is still the answer.

Worth separating that from lean-mass preservation, which this thread keeps folding into the same question. They behave differently and the advice does not transfer.

Following on from hank_denver — and this may be the naive question:

What fraction of loss being lean mass is actually normal, because the numbers quoted here range from 10% to 40% and cannot all be right?

Moderator note: leaving this open. It is being argued well and the disagreement is the useful part.