I get to chat with a Google specialist at least once a quarter for my biz and I just did this quarter’s calls last week and he mentioned shared budget to me. As with all of Google Ads, everything has this period of learning because regardless of the data it already has on the ads, it is now trying different strategies with the budget and being that it was only a couple days ago, your performance may gradually go up over the coming days/week. That being said, basically any new features Google introduces, it has flaws and kinks and it will always cost extra budget when changing many of the different settings in the backend, especiallly after running an ad for awhile.
I ended up not changing our settings. I tend to not change settings after ads are performing. If I get extra budget to test something new, I create a new ad but maintain the original. Hope this helps.