As it appears from this local log
https://logkeeper.mongodb.org/build/d2202d47dbba7bf20ff65c834f624ce6/test/5991f5df90413033c907c9a7

assert.doesNotThrow(function() {
        coll.explain("allPlansExecution").findAndModify({query: {a: {$gte: 1}}, remove: true});
    });

this line throws if run under causally_consistent_jscore_passthrough and it means that the
$readPreference is added to the plan by shell code.

I verified that that Mongo.prototype._injectAfterClusterTime does not inject $readPreference it happens eariler, so the explain_multi_plan.js is blacklisted from the causally_consistent_jscore_passthrough.