If you check out the documentation, you’ll see a section marked “system caveats”, which notes that you CANNOT play two montages at once. If you start playing a second montage it will kill the first one’s playback immediately.
The best way would probably be to move your arm/swing animations to separate, single anim instances, and then use them to blend. You can take your “primed” sections and make them separate anims, and use notifies instead of branch points.
So, for example, you could have the prime-up play On Pressed (making sure you don’t set it to Looped, so it rests in the “primed” position), and then have a notify at the last frame. The notify event can run, in the event graph, to a gate which is opened by On Released and closed by On Pressed, which fires the actual swing.
Or, you know, however your melee system actually works, I don’t know how precisely you want to handle releasing the key before being “primed” for instance. Point is, this driving logic fires the separate anims in the normal way (bools in the anim graph) and then you do a layered blend per bone to play them back.