@hwz Looks like this is a non issue. I have an email from ads.twitter.com support desk that describes the issue as such:
In general, when the cost per install bidding is enabled in a campaign, the daily budget cannot be reduced anymore, as the campaign has already been optimized to spend through the selected budget. If you really need to reduce budget, we would advise you to pause the campaign and set up a new one with a lower budget.
Hope this helps people that are trying to create CPI campaigns and attempting to decrease budget.
NOTE: You can increase budget if your campaign is doing well, so this is not a terrible restriction.