No worries 
There’s a lot of stuff going on right now anyways - hope you’re keeping well.
I’m assuming you’re familiar with Attack Decay Sustain Release?
Rather than fabricating a Release by using the fadeout (which will still be calling the cue when you press the key again) try splitting the sound up so that the release is a separate cue that can be called when the key is released.
You could also take a look at the sound cues in the UE4 Shooter Game - the one with the hovercars. It’s got a rifle in it that uses a single-shot sound, a looping sound, and a ‘Shot End’ cue that basically puts the tail onto the looping parts.
You could try using what’s called a round robin: synths use this so that they aren’t loading the same sound each time, and so the listener can’t tell so easily that they’re hearing a synthesised sound. If you create three or four or more versions of each note and assign each one their own cue you could cycle through them in a Blueprint. That means you’re not calling the same cue each time but you’ll get the same note.
If your Player pressed the same key they’d loop through the round robin.
Another thing to consider is concurrency. You can use this in UE4 to limit how many times a sound cue can be called. All electric keyboards have this feature to help manage the sound and reduce the risk of artefacts. If you bring up the console while you’re playing the game and use the Stat Audio command you’ll be able to check how many cues are playing at once. If there’s a lot of demand on one cue that might be the issue