This is reported as defect DE42548. The session handling issue is resolved in 15.5.1 which is available and patch 126.96.36.199 targeted for FEB 2019.
If this is occurring in the SaaS environment, the may be able to be alleviated by having the F5 OneConnect feature enabled.This OneConnect Profile feature can increase network throughput by efficiently managing connections created between the BIG-IP system and back-end pool members. The OneConnect feature works with HTTP Keep-Alives to allow the BIG-IP system to minimize the number of server-side TCP connections by making existing connections available for reuse by other clients.
Please open a case to have the SaaS team enable this setting.