Editing a node changes its uid

I haven't tested that case, no.

We can just move this issue to (Drupal, forum.module) if that's where the problem turns out to be.

Lots of forum and comment functionality seems to have broken when the forms api landed, so it's very plausible that's where the problem actually is.

I just tried this:

1. Disable taxonomy_access, and unloaded the module
2. Gave 'administer nodes' permission to user role 'x'
3. Logged in as 'u' and went to a forum topic not created by 'u'
4. Moved the forum topic to a different forum

The forum topic remained owned by its original owner, as expected.

That's because the administer nodes permission is special cased in node_submit as:

  if (user_access('administer nodes')) {
    // Populate the "authored by" field.
    if ($account = user_load(array('name' => $node->name))) {
      $node->uid = $account->uid;
    }
    else {
      $node->uid = 0;
    }

    $node->created = strtotime($node->date);
  }

The place where this bug is introduced immediately follows:

  else {
    // Validate for normal users:
    $node->uid = $user->uid ? $user->uid : 0;

But the taxonomy_access 'edit' case is a third case. We don't want to take the uid from a 'name' field, nor do we want to set it to the user that did the edit. Instead, we want to want to preserve the node's pre-existing uid, whatever it was previously.