In Smath Solver, I guess -1℃ is treated as -1 × 1℃ → -1 × (273.15K + 1K) → -274.15K, not (273.15K - 1K) → 272.15K.

Is there any way to put in the negative sign, not minus?

I found the caution with negative numerical values in the Introduction Manual.
-1℃ to be put as (-1)℃ for correct calculation.

I don't know what introduction manual you refer to (there is no official one), yet this one probably would do: https://doi.org/10.25933/opus4-3370. Section 3.6 is on temperature units, section 3.5 is on frequency and angular velocity. Both they handle common pitfalls for new users.