While I haven't exactly tested everything what can work, I can tell you that when I get a kill with Ace of Spades and I run as a Warlock to reload with T. Steps, it does not trigger the Memento Mori buff, but it should.
At first I thought it was exclusive to a manual reload thing, then I reloaded as a Hunter using the dodge and it does in fact trigger the buff.
The Memento Mori description says it will trigger upon reloading, and running with Transversives is still a way to reload.
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The reload probably takes to long to trigger them most often