A shift of 2-3 beats at tempo 134 can be expected. ST3 is not a fully featured sequencer and the timing of sent midi messages can vary by a few milliseconds. That‘s usually no issue for the main purpose of sending program changes. In this case it leads to these small tempo variations.
A possible workaround could be that you create snapshots on the flow 8 with the correct times for your fx and the just recall these snapshots through ST3.