See BF-31072 for a detailed description/repro of the problem. The following pipeline (A) is ineligible for a COUNT_SCAN:
 

[{ $match: {"num": {$not: {$gt: NumberDecimal("9.999999999999999999999999999999999E+6144")}}} },
{$count: "num"}]

However, when this pipeline (B) runs first:

[{$match: {"num": {$not: {$gt: NumberDecimal("Infinity")}}}},        {$count: "num"}]

B uses a COUNT_SCAN, and generates a plan-cache entry, which A then (incorrectly) uses. Both queries should match/count an empty document {}; however, B does not count it when using a COUNT_SCAN.

This is only reproducible with SBE enabled, plan cache on, and match expression optimization enabled.