The VST Sync in Biab has never worked correctly, years of nagging has done little to fix the issue.
Here's some things to try:
1. Load Element VSTi into FL Studio
Free Plugin Chainer Element + BB VST3 Support + More Midi Tracks Win\Macdrag the BiabVST Plugin into Element, hook the ins/outs up, set the EXT to sync.
2. Load Element VSTi or VSTfx into Biab
If Biab is running as administrator you need to run the file explorer as administrator also to drag the FL-StudioVST Plugin into it, hook the ins/outs up, set the EXT to sync.
Now, because of the vst sync issue never being fixed it will only sync if play is started from the very start (count-in).
also:
A VST to send Tempo/Play/Stop/Loop/Continue to main Biab app from DAW See Full Screen Pic