If there is a space after a line in server.properties the line gets ignored and doesn't seem to throw any warnings (at least none I noticed).
he specific issue I was having was with a mod that creates flying vehicles, allow-flight=true was set but was ignored because it had a space after it (which took me forever to figure out). Minecraft should ignore or even correct such spaces (from my limited experience it should be rather easy to add a check for it).
Affects 1.17 Pre-release 4
Seems to only affect integer values:
max-players=2(space)
view-distance=4(space)
Neither throw an error, but will both be reset to default, 20 and 10, on launch.
Strings and booleans did not seem to be affected in limited testing.