The Front (rewind) button would return to the same exact integer keyframe and not allow sub-frame keys to be set.
And if they looked at After Effects key indicators. In AE a key on the side (not timeline) shows if you’re on a keyframe or not and by clicking that you can not only add a key (same as UE) but you can also delete frames just by clicking on that.
All without zooming into a single frame level increments to get rid of an errant frame, especially a sub-frame keyframe.
Rewind goes to the start of the playback range. Do you have your playback range start on a non integer frame? It’s perfectly allowable to have a non integer start frame but not something very typical and harder to do because you’d have to turn off frame snapping.
I have all the snapping options enabled. Both the go to start and go to end seems to have some variance oddly enough that even though you think it’s the same integer frame it is not. It is incredibly easy to accidentally end up with 2 keys next to each other.
The reason you know what ever you thought you keyed changes a lot when played or rewound or you notice a glow on the key.
Because Unreal lacks even a basic keyframe indicator like AE has (there is a key on this frame) and lacks the ability to delete a precise key, the only solution is to zoom way, way into the key where you find the start of the frame and partial frame have 2 different keys and you can select and delete.
The solution for Unreal is: Option to disable fractional frames. Yes, they can be occasionally useful but almost every animation-type app on the planet uses frames.
And simply add the basic UI change similar to AE.
Unreal is a powerful creation tool but it’s frustrating to have so many overlooked issues that even simple things become a time-consuming chore.