However, I have just found a non-bug situation where removing givens can actually lower the rating of a puzzle:

On the way to generating a final puzzle, my generator (which works by removing givens from a full answer) created an interim state:

- Code: Select all
`7..|5.2|498`

.45|798|321

982|314|...

-----------

3.6|.85|...

5.1|.2.|..7

4.8|.7.|...

-----------

8..|.5.|..4

15.|.3.|..9

237|849|165

Which needs a BUG+1 (SE rating 5.6) to solve.

The generator continued to remove givens and ended up with the puzzle:

- Code: Select all
`7..|5.2|.9.`

.4.|.98|32.

9..|.1.|...

-----------

..6|.85|...

5..|.2.|..7

4.8|...|...

-----------

...|.5.|..4

1..|...|...

.3.|..9|1.5

Which only needs a Type 1 Unique Rectangle (SE rating 4.5) to solve.

So there is at least one situation where "taking away squares" (for which read givens) actually makes a puzzle "easier".