There are numerous inconsistencies with Scooby-Doo on Zombie Island (1998). The most obvious is that the first movie took place in 1998 and this takes place in 2019, yet the 4 main human characters are younger than 21. This is a logical blindspot of any long-term cartoon series with a floating timeline, and allows for a bit more leeway in other matters. Most inconsistencies regarding matters such as the layout of the island, the nature of the Cat People, the fate of the pirates and their treasure, the ages of the Mystery Inc. members, etc., can be seen as deliberate examples of retconning for conformity to modern Scoobyverse continuity (see trivia). Therefore, most examples where "in the first movie they say this, but in Return they say that" are unnecessary. Each movie must be consistent within itself, however.
Much drama comes from references to the disbanding of Mystery Inc. as shown in Scooby-Doo! and the Curse of the 13th Ghost (2019), however, that movie ended with an implied promise that the agency would resurrect quickly. Yet here, the group is still defunct and determined to remain so.
While Alan was videotaping the cats attacking the crew, the cat before attacking him was brown, but was gray when doing it.
Even though never shown in Scooby-Doo on Zombie Island (1998), there could have been a guest book for Mystery Inc. to have signed off-screen. Although it was a private residence, it more operated like a bed and breakfast to help Simone lure people to the island, in which case a guest book would make sense.
The film takes place shortly after Scooby-Doo! and the Curse of the 13th Ghost (2019), and some time after an imperceptibly retconned/updated version of Scooby-Doo on Zombie Island (1998). In Curse-13th, both Velma and Fred stated that they had never met real ghosts before, but here they remember the events of Zombie Island. Although Velma rationalizes both cases to herself as hallucinations (swamp gas in this, Himalayan mold in the other), Fred seems to have accepted all along that the Island ghosts were real, making his comment about the 13 Ghosts implausible.
Either Elvira knows the anonymous sponsor and is keeping this hidden from her audience, or her show actually accepts and provides vacations from people without making a background check on them. Neither option makes a whole lot of sense.
It's never revealed what type of contest Shaggy was supposed to have entered, nor does he show any memory of having entered one. It just comes up out of the blue. The gang are also surprised by being brought back to a shockingly familiar island, as if they were given no previous insight to where they were heading. They just put their lives in the hands of some contest they just happened to see on TV.
The Sheriff says Mystery Inc. should treat themselves to a vacation, which they do almost 100% of the time anyway. In all of their shows in the past 50 years, they seem to spend more time relaxing and lollygagging than they do on actual missions.