Why does my long buildup sequence not work?

Very long buildups are rejected by the engine; only the first frame will be used. Make sure your buildups contain less than 100 frames (including shadow frames). A length of 96 frames (48 images + 48 shadows) has been confirmed to work.

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Technical details:

When loading a buildup, the engine performs the following math:
time = [General]BuildTime * 900 / (number of non-shadow buildup frames (it always assumes the buildup has shadows) );
time = floor(time); // round down to whole integers
and saves this result as a timer duration. I imagine it's used as "how long to draw each buildup frame for". If the first line of that calculation ends up as 0.99 or less, your buildup will not be drawn properly.