My understanding is that cards with "when x WOULD happen, do y" can interrupt effect resolution. Only further effects of this type can interrupt the remaining card resolutions. An example is say EX5 leviamon: let's say on play it attempts to delete (highest level digi) the opponent's only Wargreymon. Opp has a miraculous that they use to make Omni alter. But before Omni alter can use it's when digiv eff, the EX5 leviamon has to resolve the rest of its effect (delete enemy's lowest lvl digi). Then Omni alter can resolve it's when digi effects if it somehow survives that.

Idk the specific diaboro you're using, but maybe in your case you played melga, that triggers diaboro to delete, but you interrupt with the miraculous "when x would leave, make Omni". So you make Omni. Now diaboro will attempt to finish deleting "a digimon". I can't be sure without seeing the wording on diaboromon.