This issue arises with the installation of the lastest AE build, 17.5.0 x40 and it only arises if you use the Default Keyboard shortcuts set as selected in the Keyboard Shortcuts Visualizer. If you have a Personalised Keyboard Shortcuts set active and it was created prior to the latest release, you should be fine with the old Marker shortcuts.
The reason for this new behavior is that the shortcuts previously assigned to creating Comp Markers and jumping to these markers have been reassigned to select the new Camera Gizmo thingy. You can override these new, default shortcuts by going to the new 3D section in AE's Preferences Dialog Window and deselecting the checkbox which assigns the numbers to the new Cam Gizmo.
See image below -

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