When you move the key on top of another key, it replaces that other key. Since there is no handle on the left side of the key you moved, we need to add a handle there. There are 3 possibilities:

• If we used the left handle from the existing key, we'd create a cusp. That is when the handles don't make a straight line through the key, which means that the value changes abruptly. We try to avoid cusps unless you explicitly want them with the "separate" button or by holding alt when you drag a handle.
• We could avoid the cusp by creating a new left handle matching the right handle of the key you moved. This is what we currently do, shown in your screenshot.
• We could avoid the cusp by changing the right handle of the key you moved to match the angle of the old handle. Spine can't know if you want the old left handle or the right handle of the moved key, so we'd rather keep the right handle of the key you moved.

I don't think there is a bug here. Spine tries extremely hard to make the handles reasonable, no matter what operations you make. When there are multiple possible solutions, we can't always know which one you prefer, so we choose a reasonable one. If you think the one we've chosen isn't the best, then please explain why and we are open to improving it. Keep in mind the goal is to make the best decisions for the most common situations, since that's the best that can be hoped for. There will always be other situations where the decision we make isn't ideal.

When you drop a key on top of another key, you are deleting the other key and the handles it had.

In Spine 3.x there was no attempt to keep handles at the same angle through the keys. This made for a cusp where movement abruptly changes when the playback crosses the keys.

The easiest solution is to move your keys very close but not on top of the other keys. You can hold shift to move them between frames.