So was playing against eldar and the shield doesn't work. They would take one swing light or heavy and my shield would drop guard with a quick stun against me(shield still at half to 3/4 durability). I don't think I ever experienced this same issue with other races, but could have not payed as much attention.

Also on another instance those blue eldar was firing directly into my shield, soon as I dropped it to fast attack I died instantly.

Can someone explain which melee weapons negate the purpose of the shield?

Or was I experiencing a bug?

Its probably bugued like klaw, animations and most of the ork stuff. SM shield absorbs a damm high number of hits and reduces attacker´s weapon durability. Ork shield breaks after first strong attack and stuns the user, leading him into certain death. Same if you try to block autocannon or plasma shots, shield destroyed with 1 hit, owner stuned and sentenced to death. Removed it from my loadouts long ago.

You should report this issue!

And what will that do?

They will say they are aware of it and post it on a thread or say it on stream, im sure itll get fixed next year.

After they are done taking their ten vacations between now and then.