One of our teammates is seeing this again, although their automation stack is on an older version of the p4 plugin and p4d, but based on the p4java release notes it should have sufficient parallel sync support. They hit the issue where sync would run and report success in automation, however the files were not actually sync'd, and it wasn't resolved until they explicitly set the sync configuration to have parallel sync enabled, but with threads, minimum files, and minimum bytes all set to 0.
Running Jenkins 2.164.2 with P4 Plugin 1.7.4 (uses p4java 2017.2.1535715) and SCM API Plugin 2.6.3.
Client VMs (Jenkins agents) are syncing from a P4D Commit server on 2017.2.1650199.
P4V is not installed on the client VMs
Client VMs are an assortment of Windows Server 2016 (8-core, 32GB RAM) and Mac Mojave (4-core, 16GB RAM) VMs
Let me know if you need any more information