To be clear: this is new behavior in After Effects 17.5. By default the 1/2/3 keys are now assigned to the new camera navigation tools, and the 4/5/6 keys are assigned to the 3D transform gizmo modes.
To disable this behavior and return the 1-6 keys to markers, turn off the "Use 1/2/3..." option in Preferences > 3D.
We're not planning on changing this behavior, as it is an intentional change to AE's defaults and not considered a bug. However, if you dislike the defaults or would like to see other changes, please submit an idea with what you'd like to see improved here: https://adobe-video.uservoice.com/