This is occurring for us in API, as well as in the UI. It seems that when Total Budget is null, it is treated as 0. As a result, in UI as well as via API (trying to update campaigns), we seem to be unable to set a daily budget without setting a total budget as well. As might be expected, this is extremely problematic for open-ended campaigns.
I should mention, when total_budget_amount_local_micro is not set, we are unable to update daily_budget_amount_local_micro.
Anyone else having this issue?