I’m not sure about maelstrom, but I don’t think that’s true for LA + Deadly momentum because that bonus damage is just given as raw, flat damage.
E.g. I have 100% cold/acid conversion on ABB. LA procs, and I have 100 flat cold + 100 flat acid damage.
ABB will then convert the 100 flat cold to acid.
Skills which have this weird mechanic tend to have something like ‘will attack surrounding enemies in addition to the primary target’ in their tooltip description.
So other examples would be - nightfall, explosive strike, soulfire, etc.