and that managers told them to told them to avoid mentioning the ethnic dynamics.

There a huge difference between persecuting someone and then not writing his race or ethnicity into an official report and avoiding to prosecute them.

It's clear why Pakistani council members wanted to hide it, but why did the other council members let them?

From what you quoted from the report the those Pakistani council members were influential people. Just like the politicians who covered up the child abuse in Westminster also were influential people.

In general politicians also never want that scandals and tragedy under their watch get public.

At an operational level, the Police gave no priority to CSE, regarding many child victims with contempt and failing to act on their abuse as a crime.

Regarding child victims with contempt does suggest a dysfunctional police but it's not about multiculturalism.