I have not been on my road bikes in ages really and have been using indoor bikes (Wattbike,Keiser m3i) for the past 3 months and it has persisted the whole time.
So not sure it’s my set up. I think maybe more about my posture whilst riding. I think my shoulders are at my ears too often!
In that case it might be that the reach and stack are inadiquate and you need more of one / both, your arms are straight when they should be bent more to lower your position without hunching your head down between your shoulders, or your posture is to rigid / tense and you need to smash a couple of valiums before a ride..
Not sure if it'd help with the traps specifically but I've found hanging off something, totally relaxed with an overhand grip like a monkey is great for decompressing the spine and back area generally; might be worth giving it a go after a ride