It’s worth noting that a miracle is defined as “an extraordinary and welcome event that is not explicable by natural or scientific laws and is therefore attributed to a divine agency.”
The emboldened part is a textbook argumentum ad ignorantiam fallacy. Though I also see your point, that theists are also claiming any fortuitous outcome is a miracle, even when we have a natural scientific explanation. Blame Pulp Fiction for this one.
Indeed, and this is the point even if we knew what happened couldn’t be explained by natural or scientific laws, it’s still a fallacy to assume that the lack of a contrary explanation or evidence lends any credence to the claim it was divine intervention. However in all the cases I’ve seen, it comes down to subjective claims that such an event even occurred, at best propped up with an argumentum ad populum fallacy.
The first thing required is sufficient objective evidence that a miracle is even possible, and for that they would need to objectively demonstrate that a deity exists or is even possible, as well an anything supernatural. They are trying to use the claim as evidence for another claim.