Fox Engine which is now Decima

Are you sure that’s the case?

AFAIK, even though it had a non-Japanese technical lead, Fox Engine was still an in-house engine developed by Kojima Productions.

On the other hand, Netherlands-based Guerilla Games created the Decima engine for in-studio use only, however once Kojima entered relationships with Sony, he was allowed to use it for Death Stranding.

Fun fact relating to the name:

According to executive producer Angie Smets, Decima was originally known simply as “the engine” by Guerrilla employees, as there were initially no plans to publicly offer this technology to game developers outside of the company. However, the newly-forged partnership with Kojima Productions in 2016 meant that Guerrilla suddenly had to give the engine a name for marketing purposes; they then chose to name it after Dejima, the Japanese island where a Dutch Empire trading post appeared in the 17th century and once symbolized the strong trade relations between Japan and the Netherlands.