The occupation both gave the german language a word: "Amokläufer", and caused the US army to switch from .38 to .45 caliber sidearms.

Do you have a source on the relation of “Amokläufer” to the Spanish-American war?

All the sources I can find claim it originates from Malay/Indonesian culture.

You're right, now that I look into it there doesn't seem to be anything more than circumstantial happenstance.

https://en.wikipedia.org/wiki/Philippine–American_War

https://www.dwds.de/wb/Amok "amok ‘rasend, wütend, verzweifelt (im Kampf)’, im 19. Jh. vor allem in Lexika bezeugt, im 20. Jh. auch häufiger in literarischen Werken. Ein Frühbeleg findet sich (um 1660) in einer Reisebeschreibung („Amoc welches so viel ist als courage, oder schlag todt“). Amoklauf m. Amokläufer m. (Anfang 20. Jh.)." vgl die Wortverlaufskurve ab 1600.

Checking https://en.wikipedia.org/wiki/List_of_wars:_1800–1899 and https://en.wikipedia.org/wiki/List_of_wars:_1900–1944 reveals the Semantan War, the Mat Sellah Rebellion, the South Sulawesi expedition, the 1906 and 1908 Dutch Interventions in Bali, and various too-small-for-wikipedia Dutch-Indonesian actions as possible other popularisers in the 1890-1910 period.

There was an interesting discussion of the word "amok" with Vincent Bevins on "The Red Nation" podcast last week, starting about 32:30 from the beginning. [0] Surprise! It was largely colonialist bullshit, even in Indonesia.

[0] https://soundcloud.com/therednationpod/the-jakarta-method