I am a German native speaker. But we can resolve this confusion around "spontaneously" by going to its actual origin: The English word "spontaneous" is a loan

I am a German native speaker. But we can resolve this confusion around "spontaneously" by going to its actual origin: The English word "spontaneous" is a loan word and originates from the Latin "spontaneus" which itself derives from the Latin noun "spons" (feminine) primarily meaning "free will". It also has a secondary connotation meaning "impulse". [1] So the common English usage is closer to how the Romans used the word. That does make sense: The Romans conquered the British isles but failed to win against the Barbarians that lived east of the river Rhine. The Germans only much later incorporated Latin vocabulary and did so mostly only in elite circles. That's possibly why "spontan" means "acting out of impulse". [1] https://en.wiktionary.org/wiki/spons#Latin https://en.wiktionary.org/wiki/spons#Latin