Thanks for the kind offer, here's a detailed report and an example scene (the video in OP shows the same thing, only with different frame numbers):
Scene contents:
One cube that has a Motion System tag applied to it.
The tag contains the unchangeable "Default Layer" (it can't be muted, soloed, or changed the strength of - it's always there).
This layer has three keyframes on frames 0, 20, and 40.
The tag contains a second layer, "Layer 1", which is a new layer, it can be disabled, renamed, etc. There can be many of those.
This layer also has three keyframes, but on frames 2, 10, and 18.
In the tag, the user can switch between different layers, and animating them adds keyframes to that layer.
Expected behaviour:
When "Default Layer" is active, using Go to Next/Previous Key switches between frames 0, 20, and 40.
When "Layer 1" is active, using Go to Next/Previous Key switches between frames 2, 10, and 18.
Actual behaviour:
When "Default Layer" is active, using Go to Next/Previous Key switches between frames 0, 20, and 40. (correct)
When "Layer 1" is active, using Go to Next/Previous Key also switches between frames 0, 20, and 40. (incorrect)
The issue is that the Go to Key commands don't take into account the currently active motion layer (in this case, "Layer 1") but only use the Default layer. The timeline still shows the currently selected motion layer's keyframes, so using the Go to Key commands appears to jump to random frames.
example.c4d