Since the introduction of the Multi-Mongo Mongo, the core suites tests will dispatch randomly commands and queries against any of the mongos deployed following the server selection specs of drivers.
In SERVER-114498 we started deploying 2 mongos for the sharding_jscore_passthrough family, which found out that rarely (but kinda consistently at every evergreen patch) $planCacheStats might report empty result when it's expected not to. 

Some failed patches listed:

• patch 1
• patch 2
• patch 3

To bypass the issue, the Multi-Router is currently redirecting all the commands and queries belonging to any test that contains "plan_cache" in the name against the same mongos (as it used to be before), which seems to fix the issue.
There is a high chance this is a correctness issue.

in this logs we can see all the finds correctly reported 1 document, but the subsequent $planCacheStats didn't

Once fixed, address the TODOs related to the ticket