Submitted by hansfriese
https://github.com/code-423n4/2023-02-malt/blob/main/contracts/RewardSystem/RewardThrottle.sol#L445-L455 
https://github.com/code-423n4/2023-02-malt/blob/main/contracts/RewardSystem/RewardThrottle.sol#L576
RewardThrottle.checkRewardUnderflow() might calculate the cumulative APRs for epochs wrongly.
As a result, cashflowAverageApr will be calculated incorrectly in updateDesiredAPR(), and targetAPR would be changed unexpectedly.
In checkRewardUnderflow(), it calls a _sendToDistributor() function to update cumulative APRs after requesting some capitals from the overflow pool.
The main reason for this issue is that _sendToDistributor() doesnt update the cumulative APRs when amount == 0 and the below scenario would be possible.
As a result, the cumulative APRs will save smaller values from epoch 102 and cashflowAverageApr will be smaller also if the smoothingPeriod contains such epochs in updateDesiredAPR().
So the updateDesiredAPR() function will change the targetAPR using the smaller average value and the smoothing logic wouldnt work as expected.
I think _sendToDistributor() should update the cumulative APRs as well when amount == 0.
0xScotch (Malt) confirmed and commented:
