The reason last updated is changed when a user monitors an issue is to reflect the continued interest in the issue and bubble the issue to the top, where developers would see it again. This is similar to a "ping!" feature.

However, I agree that this behavior could be made configurable.

I think my biggest issues here are:

• Normal users don't see why an issue that was seemingly updated a long time ago is at the top of the issue tracker (they don't see the start/end monitor entries in the history), which looks inconsistent.
• End monitor updates the issue, which leads to a lot of noise in the history in particular if this happens after the fact that the issue was already closed. Many users seem to remove the monitor after an issue has been resolved to keep their monitor list tidy, and I don't see why it's necessary to bring an issue to the top of the recently modified list for this action.

I also found it strange that the “Last Update” field in an issue changes even though nothing in the issue itself has changed, except for the list of watchers. From a technical point of view, the issues table in the database hasn’t changed either.

PR: https://github.com/mantisbt/mantisbt/pull/2213

If the “Ping! ” feature is needed, it’s easy to implement using a plugin.

Why this PR is important to me: I’m trying to develop a plugin (https://github.com/raspopov/MantisChronoTag) to automatically manage issues by assigning them special tags that trigger different actions at specific times. To achieve this, I need to track a baseline timestamp, and the 'Last Update' field is ideal for this purpose — I really don't want to expand the database schema. The current uncontrolled modification of this attribute by issue observers also makes life very difficult for the plugin.

If the “Ping! ” feature is needed, it’s easy to implement using a plugin.

Maybe no longer needed.
When this issue was created, there was no Timeline on My View page.
The Timeline shows users un-/monitoring issues.

BTW on the view_all_bug_page.php page, the default sort order is by the “Updated” column, which is affected by observers.