Yes—this is a known behavior with the MiniTT1. The AF-assist signal may not pass through or activate immediately the way it does when the flash is mounted directly on the camera. Community reports indicate that the AF-assist can start working only after taking a test shot first.

So if you need the assist beam, try firing a shot after mounting the flash on the MiniTT1, then half-press focus again. If it still doesn’t work, the practical takeaway is that AF-assist through the trigger is not as reliable as direct hot-shoe mounting.

In short: your trigger is likely functioning normally for E-TTL, but AF-assist behavior through the MiniTT1 is limited/quirky compared with putting the flash directly on the camera.