Mod Delay set to sync makes a click in the audio when the tempo changes

I have several PT HDX systems - on all of them (and on this laptop) a tempo change makes a click in the audio if I use a mod delay set to sync.

The file attached is a 1k tone sent to a mod delay. After a few seconds the tempo changes from 120bpm to 50 bpm, and there's a nasty click in the audio.