There’s a couple of reasons why “Optimize” would change everything around.
If you’re pressing “optimize” and getting vastly different results without changing anything, the most likely explanation is that there’s something the plan can’t quite figure out, and it’s bouncing between options that look equal to it.
As an extreme example, consider a touring plan where there’s no way you’ll be able to finish everything in time for dinner at 6 p.m. In that case, every option the Optimizer looks at will be considered a failure - everything looks equally bad, because nothing works. In that case, the Optimizer will probably just keep shuffling things around.
The other reason this might happen - and this is subtle - is that the Optimizer imposes small, hidden penalties when a plan cuts it too close to show, meal, or break times. For example, the Optimizer doesn’t want you to get to a show 1 minute before the show starts - it wants to allow plenty of time for you to get there and find a seat.
When you arrive at a show within 15 minutes of the scheduled start, the Optimizer adds a small number of penalty minutes to the plan. The closer you’re cutting it, the more penalty minutes it adds.
Those penalty minutes add up. If it happens across a few shows, meals, or breaks, there may be many alternative plans that all look equally good within that number of penalty minutes. (That is, if it can’t avoid the penalty, then there are lots of plan alternatives with about the same total penalty minutes. It may be trying those.)
That’s the other way we could be making the thing you’re seeing. Post the URL to the plan if you want me to take a look at it though.