forum.freeciv.org

* Each worked tile that gives more than 2 Food, Shield, or Trade will suffer a -1 penalty, unless the city working it is celebrating. (Cities below size 3 will not celebrate.)

- 1.10 My irrigated grassland produces only 2 food. Is this a bug?

No, it isn't. It's a feature. Your government is probably despotism,
which has a -1 output whenever a tile produces more than 2 units of
food/production/trade. You should change your government (see
Government) to get rid of this penalty.

Under Despotism or Tribal governments, the Pyramids cancel the penalty to worked tiles with output greater than 2 (for all kinds of output), as if all your cities were always celebrating.