Yeah I've had the same issue, once I get 100K+, the game suddenly gives much less. It looks like there's an overflow and the number resets to 0.
Viewing post in Incremental Booster Packs comments
I think the high number of spawning entities gets fully calculated for the display value when you trigger a long long cascade. However, the only ones truly captured in terms of profit are those that load in and are destroyed prior to the completion of the level, or before the lag forces the calculation to halt through some error in the way it tries to handle that calculation.
Clearly the way the display score is tracked and updated is different than the way the final profit is tracked and calculated, so it may be a simple fix of just using the properly tracked variable since the display score does appear to be correct.