Intermediate CA trust store gap

Internal service B presents a client cert to service A over mTLS. At 10:00, after B's cert was reissued by a new internal sub-CA overnight, A starts rejecting B's handshakes with 'unable to verify the first certificate' / 'certificate signed by unknown authority', but only A to B; B's calls to other services that trust the new sub-CA's root succeed. Dashboards: B's new leaf is valid and unexpired; A's trust store contains the ROOT CA. No clock issues. How do you triage and mitigate?