Yes, loss of coverage caused by a reduction in hours is a COBRA qualifying event. That's also true if the reduction in hours causes the loss of coverage at the end of the applicable ACA look-back measurement method stability period.
More details: https://www.newfront.com/blog/the-cobra-triggering-events
It doesn't matter whether the employee is eligible for active coverage at that point. By definition they aren't be eligible for active coverage, otherwise they wouldn't have lost it and had the qualifying event. In other words, basically everyone on COBRA is no longer eligible to enroll in active coverage at open enrollment.
Here's a quick overview--
https://www.newfront.com/blog/loss-of-health-plan-eligibility-caused-by-move-to-part-time-work
Employees who do not average at least 30 hours of service over the full standard measurement period (i.e., generally do not reach 1,560 hours of service in the typical 12-month standard measurement period) can be removed from coverage as of the start of the new stability period (generally the start of the new plan year) because the employee will be treated as part-time for ACA purposes for the duration of that stability period. This will be a COBRA qualifying event as of the end of the plan year in which the employee loses coverage (loss of coverage caused by a reduction in hours).