The latest primeng v12.0.1 component templates (Angular .component.html file) are not yet updated to the latest primeflex v3 classes which halts the applications using primeflex and primeng to make use of primeflex v3.
Note: I have tagged this issue as "Fixed" for now. If a new update comes from your side, please reopen this ticket. Thanks a lot for your understanding!
Best Regards,