Generally, a novelization of a preexisting script (such as all of the novels that ADF is complaining about) is a "commissioned" work, meaning that it is a WFH under American law. As a WFH, the commissioning employer would own the copyrights, and an author would not be entitled to royalties because they aren't licensing a copyright from them. To the extent the author might receive any ongoing payments, those would generally be "performance bonuses" for reaching milestone sales. It is extremely rare for commissioned works to be subject to royalties, and that generally involves novelizations.

Interestingly, ADF is claiming that the payments are royalties. (https://www.theverge.com/2020/11/19/21578621/disney-alan-dea...) Given Fox's history of non-standard/talent-friendly legal practices during the era in question (see, e.g., letting Lucas keep the rights to Star Wars), that sounds like something that 80's era Fox would have done...with respect to the Aliens novelizations.

Something is off with how both sides are characterizing the agreements. Disney claims it didn't acquire any liabilites--but both the Fox and Lucasfilm acquisitions were stock acquisitions, meaning that acquired companies inclusive of all assets and liabilities. And ADF is claiming that WFH agreements included royalties, which is extremely rare.

Ultimately, unless someone coughs up a contract, there's no way to be sure who's right here.