It's a known problem, yes. Unless that is a new problem, however, lower crime rates are still lower crime rates. If you're arguing that under-reporting is a new or dramatically worse problem, how does switching in 'tough on crime' politicians fix under-reporting? Under-reporting is a police department policy that makes cops look good, so why would they stop doing it? If you want to fix that problem in particular, you should vote someone in who will fix it, not someone who wants to throw more people into the local lock-up.
If cops have been throwing out 50% of reports forever, then 200% of a lower number is still less than 200% of a bigger number from past statistics.
The degree of under reporting can and does change through time. If you have a pro crime pro criminal DA like Chesa Boudin people respond to that. People report less crimes if they know nothing will be done. If they know stopping crime isn't a priority police are less likley to do anything to stop it.
People respond to incentives. Incentives vary over time.
If cops have been throwing out 50% of reports forever, then 200% of a lower number is still less than 200% of a bigger number from past statistics.