On 30 Jan 2018, at 19:05, Wolfgang Schuster <schuster.wolfgang@gmail.com> wrote:
[…]
Spaces after a control sequence are gobbled and multiple spaces are skipped.

Braces around the filename are allowed because this function was added with the extensions to the TeX binary (which means it doesn’t work with Knuths original TeX), try this \input{ knuth.tex} to load a file with a space in the name.

Yes, I understand now the rationale behind this behavior.
Many thanks!