Hey there,
1) Tbh this needs a closer look at the workflow but here is what I would do to find the root cause:
Go to workflow and check the actions log tab, there you should see what exactly the workflow is doing, and from there check what happened to the date property of a certain ticket. Seems like your branches are able to edit the same property, which can cause problems, also keep in mind data processing with workflows can take some time which might be problematic when you have two branches working with the same properties.
If your ticket stages can change fast, you can think about adding short time delays within your workflow to allow automation to fully execute before moving to the next action or branch.
2) I have not experienced such thing but you can check what your calculated property show when you do report on that? Maybe it’s a front-end issue that you are seeing it this way, if its still the same when you report, and you are sure your logged dates and calculated property logic is right, I would suggest checking it with support.
Again it needs a closer look I think but hope it helps
Cheers