off by one error when updating indices after mutation

I can't repro this in an isolated case but there is an off-by-one error for a combination of an
`path.insertBefore(...nodes)` and `path.replace()` (deletion). I worked around by moving from:

```
// path.insertBefore have called multiple times before this.
path.insertBefore(...nodes);
path.replace();
```

to

```
// path.insertBefore have called multiple times before this.
path.replace(...nodes);
```

I'll keep trying to repro in isolated code but I wanted to report the issue incase you have any ideas

No file or test is named. Start by building a minimal reproduction around repeated path.insertBefore(...nodes) followed by path.replace(), compare it with path.replace(...nodes), and trace the index updates during mutation. Done means the off-by-one behavior is reproduced and the relevant update path has a verified regression test.

Written by the indexing model from the issue text.