Sweeping !64205 from 21.0 to 21.2. [ATR-27650] Perform double-merge of trigger menu metadata during POOL->POOL transforms, fix for MP shared writer.

Atlas Nightlybuildrequested to merge
cherry-pick-81597e4fb70-21.2 into 21.2

[ATR-27650] Perform double-merge of trigger menu metadata during POOL->POOL transforms, fix for MP shared writer.

See merge request !64205 (merged)

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