The documentation of --cap-drop states:
By default no caps are left in the sandboxed process.
That seems not to be true:
id
uid=0(root) gid=0(root) groups=0(root)
getpcaps $$
14257: =ep
bwrap --bind / / sh -c 'getpcaps $$'
15598: =ep
bwrap --bind / / --cap-drop ALL sh -c 'getpcaps $$'
15577: =