Unexpected sync groups behavior

[maslenok]

Yes. I solve this issue.

1. In animModifier class i procuderally create two curves for each sync group ( 1. Anim length value, 2. Weight curve (value = 1)) for each sync group. Curves must be created in all synced anims.
2. After that convert all synced anims to single frame mode (right mouse click on anim node).
3. In anim BP create float var to drive anim time for sync group. My example on first screen
   
4. connect variable to each anim node. Note: for blendspaces u dont need scale value to anim length, it drives by normalized time position
   
5. Use “modify curves” node for filter sync group curves at any anim graph position (right side on second screen)
6. If u have sync markers - delete all. Because it add unexpected behaviour to single frame anim nodes.

For your case with start stop transitions you can manually modify sync group variable as you want.