There seems to be a bug in Cadabra v1.x. (I've checked in 1.42 and 1.46).
The \sqrt function seems to drop numerical prefactors in the displayed expression (and in the .cnb file) when no other objects multiply it.
Consider: The first expression returns \sqrt{x} but the second returns 7 y \sqrt{x}.
7 \sqrt{x};
7 y \sqrt{x};
This seems to be partly a bug in how the expression is displayed. The first expression above is rendered as "\sqrt{x}" in the .cnb file but in expressions it seems to be treated correctly as 7 \sqrt{x}, so that, for example, "7 \sqrt{x} - 6 \sqrt{x}" displays "\sqrt{x}" (after collecting terms) but "7 \sqrt{x} - 7 \sqrt{x}" returns zero.