In Python, at least, the built-in title method doesn't quite do the job here

I am not sure what exactly you mean by "partial messages", but there is this note in the description:

NOTE: Extra spaces before or after the code have no meaning and should be ignored.

Doesn't it says at least 3 numbers there? Meaning three or more? IMHO you're saying the same, but I can be wrong and maybe one way of writing it is clearer than the other.

It is not considered a range unless it spans at least 3 numbers

That means [1, 2, 4, 5, 6] is "1,2,4-6" and not "1-2,4-6".

None of the consecutive elements at positions 0,1, and 2 differ by more than 1

-10, -9 and -8 form this range: "-10--8". I think you're confusing what should be consecutive as a number and as an element in the list.

solution([-10, -9, -8, -6, -3, -2, -1, 0, 1, 3, 4, 5, 7, 8, 9, 10, 11, 14, 15, 17, 18, 19, 20]);
          |         |   |  |              |  |     |  |             |   |   |  |            |
          -----------   |  ----------------  -------  ---------------   |   |  --------------
            "-10--8   ,-6,         -3-1,       3-5,         7-11,      14, 15,      17-20"

7-11 starts out well, 1 position after 3-5, but then it swallows a whole 5 numbers, and abandons 15.

11 and 14 aren't consecutive numbers, so, obviously, the range ends in 11.

The range 17-20 fortunately heralds the end of this hamster wheel of reasoning, but not before mysteriously consuming four numbers.

There is no mistery there, it says at least 3 numbers, there can be more.

I'd close this, because I think it all comes to your misunderstanding of the instructions, which are fine be me.

The goal is to group all numbers adjacent in the list which differ at most by 1. In your example of [-9, -8, -6], two last numbers do not differ by 1. If you have an idea for a better wording, feel free to suggest one.