We have investigated, and determined that this problem only occurs if you are opening an existing worksheet AND that worksheet contains a restart command. It does not affect new worksheets, or existing worksheets that do not include a restart.
We are working on a fix now. In the meantime, a partial workaround is to remove the restart commands whenever possible. We recognize that this solution will not be an option for everyone, but note that if the restart only occurs at the beginning of the document, it can be safely removed. By default, each worksheet starts with a new math engine, so an initial restart is not necessary.
We apologize for the inconvenience, and will let you know when we have further updates.