Nested numbered lists are not serialized correctly

Note: The affect version is inaccurate and must be refined.

Reproduction steps

1. Create a numbered list with 3 items: A, B, and C
2. Select B and C then click "next block" on the floating toolbar
3. Observe the serialized markdown

Expected

1. A
  1. B
  1. C

Actual

See how B is wrongly indented.

1. A
1. B
   2. C