There is a bug in which the override frequency is not generated even when combining the resonant stem. Due to this bug, 20 resonate stems are lost. Please check this bug.
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Bungie is already aware of this issue and working on it. https://www.bungie.net/en/Forums/Post/254628234?sort=0&page=0 [quote]Combining four Resonate Stems is not awarding an Override Frequency.[/quote]