Anchor negative offset validation fails to re-run in some cases

[dandelany]

While testing #1822 with @AaronPlave we noticed an issue with anchor validation not re-running in some cases. This seems to be a pre-existing bug not directly related to #1822 - to reproduce:

1. Create an activity ("Activity A") on a plan, within plan bounds
2. Create another activity ("B") and anchor it to the end time of Activity A
3. Set the time offset of Activity B to a small negative interval, like 1 second. This is invalid/not allowed, & you should see an error message ("has a net negative offset relative to an end-time anchor").
4. Move the start time of Activity A (the anchor parent) slightly
5. Observe that the expected validation error on Activity B has now gone away

This is relatively low priority if this is the only case where this happens - however we should investigate and identify if this occurs for other types of validation errors too, or just this.

[Mythicaeda]

I'd like to correct this bug: I'm fairly certain the only way for the validation message to go away is if the validation is rerun, meaning something about the changed state is making the rerun fail to identify an issue.

Aside from that, I have a couple of clarifying questions for reproducing this:

1. Was this observed in the UI or DB? I'm presuming both, but I want to confirm since I imagine you saw this during some manual integration tests
2. What was the initial relationship between A and B's offset? That is, was the magnitude of A's offset greater than, less than, or equal to B's offset?
3. Can you define how the start time of Activity A was moved? Was it moved closer to plan start? Further? Was the amount greater than, less than, or equal to the magnitude of B's offset?