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Both effects are type-changing effects that apply in same layer
613.8. Within a layer or sublayer, determining which order effects are applied in is sometimes done using a dependency system. If a dependency exists, it will override the timestamp system.
613.8a An effect is said to “depend on” another if (a) it’s applied in the same layer (and, if applicable, sublayer) as the other effect; (b) applying the other would change the text or the existence of the first effect, what it applies to, or what it does to any of the things it applies to; and (c) neither effect is from a characteristic-defining ability or both effects are from characteristic-defining abilities. Otherwise, the effect is considered to be independent of the other effect.
It appears to be an uncommon case where to find dependency one has to check result of ability, not only if it still exists or what it applies to. Since applying Fog changes what Grond ability ends up doing, it should be considered dependency and Fog should override Orc Army types before Grond checks them.
Or at least I see no interpretation of those rules that does not result in dependency existing.
Parse-wise, the bug here is that this is the first time I'm aware of for Arena where the dependency exists because of some "distant" clause in the rules text - the parser didn't see the contents of the "as long as" clause when considering what may be relevant for dependencies for the "Grond, the Gatebreaker is an artifact creature" phrase. The sorts of dependencies we've handled in the past are those like [[Ashaya, Soul of the Wild]], where the "required" types (Nontoken creatures you control) are in the same phrase as the "dependent" types (Forest lands).
We hope to get the fix out for this dependency issue soon. #wotc_staff