Previously, sorting in Signals Notebook used to be natural (https://en.wikipedia.org/wiki/Natural_sort_order): MP-12, MP-99, MP-134. Now it's alphabetical: MP-12, MP-134, MP-99.
In Individual Edition, users can name their experiments whatever they want. So a user could have the following experiments: 1, 02, 3-1, MyNewIdea, Brian-4. This is an extreme example, but there is no guarantee that the names can even be sortable numerically.
WORKAROUND 1:
Sorting by "Created" may serve you as a workaround, should you always name your experiments sequentially.
WORKAROUND 2:
You may want to implement padding zeroes in front of the numbers when naming your future experiments, e.g. "MP-001-00001". You can choose the number of zeroes yourself, so that the maximum number in your format will not be reached based on your current data creation rate. The padded format should work as you expect regardless of alphabetical vs natural sorting future in the Signals Notebook. It may also provide a better sorting experience should your data be used outside Signals Notebook.
In Standard Edition, administrator can set naming rules that will satisfy various needs, including sorting requirements in Signals Notebook and outside. Please check with your tenant administrator if your organization wants to use that capability and enforce naming rules in Signals Notebook configuration. From more information, your administrator can refer to our System Configuration Guide > System Objects > Experiment Settings > Experiment Auto-Numbering.
There is a related post on our Ideas Portal (which you can access from the Communities menu at the top of this page). Search for SNB-I-87. Feel free to vote/comment/share.