Thanks for taking the time to comment. Did you implement it? How'd it work for you?

From Odoo's point of view, this is a feature and not a bug.

I pushed this question to a support ticket. Here's what I got back from nikp@odoo.com

According to Odoo standards, backorders are generated only when a manufacturing order is planned, and a portion of the product quantity is registered before initiating any operations. If the production process is already underway and the product quantity is adjusted during the ongoing operation, a backorder will not be created for the remaining quantity at that time. Instead, the backorder is created only after the entire manufacturing order is completed. This functionality is in line with Odoo's standard behavior.

Note: none of this is documented.

From my 2 cents, backorders should happen any step.

From what I understand, all WOs share the same register production step data. So, by registering production in 1 WO, you do it for all WO. it is possible to overwrite it from subsequent WOs.
I was actually having the opposite problem. I didn't want any backorder to be generated from the WO, as our processes are not able to produce precise and predictable quantities.
My guess would be that removing the self.is_first_started_wo would allow any WO to create a backorder, but that might create duplicates if it's triggered on every WO in the MO.