Every 2nd frame turns black after 50% slow down

In the timeline, I insert one 20 s 30 fps clip (with transparent areas, Apple proRes 4444, and generated from Motion).

Then I double the length of the clip to 40 s by slowing it down to 50 %.

Then I add below a plain colour Custom Solids Generator which I extend to the full 40 s.

No matter the generator, most of the time, the clip transparent area turns black every second frame. Weird enough as it sounds, rendering the clip makes it worse every time.

Removing the 50 % slow down solves the problem, but that does not help me.

Any idea what I am doing wrong here?

FCP 10.8.1

Motion 5.9

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Br