The "Compound Arithmetic" effect does not work with the "Copy" operator selected inside of mogrts when imported into Premiere. The "Compound Arithmetic" effect is included in AE only for legacy support, as per the documentation; my suggestion is that this effect is moved into the "Obsolete" category to ensure that beginner users are deterred from utilizing this effect, given that it does not work properly with some of After Effect's more recent features.
For those looking for a workaround to copy footage from one layer to another (which is extremely useful within mogrts), I recommend using the "Set Channels" effect instead, which does work within mogrts imported into Premiere.
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