The two statements sound logically contradictory to me:

1) I impose an 80% CO2 reduction for a future year.

2) The emission sum from the result is actually greater than the imposed cap value.

Therefore, I suspect that your constraint is not imposing the emission cap correctly. Because if it were correctly modelled, the emissions would be bounded by the cap value. However, as I have no access to your model, it is not possible for me to see which flaws your constraint may have. One frequent beginner's error is forgetting to use an interpolation option for the RHS, if it is not specified as a full time-series, just to mention one out of numerous possibilities.

1) I impose an 80% CO2 reduction for a future year.

2) The emission sum from the result is actually greater than the imposed cap value.

Therefore, I suspect that your constraint is not imposing the emission cap correctly. Because if it were correctly modelled, the emissions would be bounded by the cap value. However, as I have no access to your model, it is not possible for me to see which flaws your constraint may have. One frequent beginner's error is forgetting to use an interpolation option for the RHS, if it is not specified as a full time-series, just to mention one out of numerous possibilities.