As discussed in SERVER-89579 it is expected that FCBIS can result in incorrect fast counts.

validate has an enforceFastCount option we can set to false to skip checking fast count in these cases, and we can enable it for any relevant tests using TestData.skipEnforceFastCountOnValidate.

We use this for some FCBIS tests now but there are some cases we've missed e.g. BF-33936.

SERVER-90010 disabled validation altogether when using FCBIS but we should probably revert that and switch to just using this option instead so we still get the other benefits of validation.
We can consider an approach similar to SERVER-90010 though where we check if the node is using FCBIS, and if so set the option automatically, rather than having to set it individually for tests/suites.