Why is my lemon curd metallic? How should I fix it?
From your Edit, it looks like you have narrowed the problem down to the egg yolks.

Did anyone else taste your original batch of lemon curd? If you still have original batch, you may have a close friend or relative see if they can taste difference.  For years (side effect of medication) I would perceive either "off or metallic taste" which my husband or anyone else could not taste.  Pregnant women will also experience metallic taste (dysgeusia):


  https://www.whattoexpect.com/pregnancy/symptoms-and-solutions/metallic-taste.aspx


Off taste from yolks are usually associated with "old eggs."  It is usually described as tasting like sulphur.

What chickens eat also affect the taste of the eggs:


  https://grist.org/urban-agriculture/2011-11-1-peebottle-farms-what-to-feed-your-chickens/
  archived at: http://archive.is/WccwR
  
  http://homesteadingguide.com/115/news/best-tasting-eggs-what-to-feed-your-chickens/
  archived at: http://archive.is/rt2RF
  
  The following post has made me shy away from any "natural feeds" which
  contain kelp:
  http://www.maryjanesfarm.org/snitz/topic.asp?TOPIC_ID=64661   archived
  at: http://archive.is/EUac3