The differences due to regions, RCPs and timeframes are related to the current temperature level and degree of warming . The projected effects of climate change are positive where current annual mean temperatures  are below 10℃, but they become negative with Tave above around 15ºC. At Tave > 20ºC, even a small degree of warming could result in adverse effects. In maize, negative effects are apparent at almost all temperature zones. A new study using the latest climate scenarios  and global gridded crop model ensemble projected that climate change impacts on major crop yields appear sooner than previously anticipated, mainly because of warmer climate projections and improved crop model sensitivities .
IPCC
Climate Change 2022: Impacts, Adaptation and Vulnerability
IPCC Report