If you experience strange play/reset behaviors with an external clock, you’re likely running with a small clock ratio, like 1:2, 1:4, and so on.
USTA’s time ratio mechanism works exactly like a digital clock divider or multiplier. If you use small time ratios, you are multiplying the clock. A clock multiplier creates more clocks than it receives, so it needs a steady stream of impulses to calculate the timing.
You have three possible options:
Use a clock-to-unit ratio of 1:1 whenever you use an external clock (recommended); Use lower ratios with the internal clock only; Play a sequence once before the “official” playback to allow USTA to multiply the external clock correctly.