The $_internalConvertBucketIndexStats stage used PauseExecution as a way to signal "skip this document" when an index stats conversion failed. But PauseExecution is not a general purpose skip mechanism, but rather a TeeBuffer-internal signal used solely by $facet to coordinate its sub-pipelines. When this stage is placed before $facet in a pipeline, TeeBuffer receives the unexpected PauseExecution from upstream and hits a hard invariant assertion, crashing mongod.
Exploitability
AV:NAC:LAT:NPR:LUI:NVulnerable System
VC:NVI:NVA:HSubsequent System
SC:NSI:NSA:N7.1/CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:NOther