There's a short [Sina blog post][1] that contains some explanations.

There was a《普通话异读词审音表》that was released in 1985. One of the words on the list was 帧:

>帧。“装帧”指书画等物的装潢设计。《集韵·映韵》：“帧，张画绘也。”猪孟切，字书据此注为zhèn。声旁是贞，通行的读音是zhēn。《初稿》未审，《定稿》读为zhēn，与贞、侦读音一致。

On the same document you can also find 荨:

>荨。《初稿》审订“荨麻”一词，定“荨”读qián，不取xún音。“荨麻”是一种植物，人如果被它刺伤，皮肤就刺痒起疹，成为“荨麻疹”。医生和病人把“荨麻疹”读为xúnmázhěn，与“荨麻”的读音不一致。《定稿》把“荨”字的读音分为文读和白读：文读读qián，如荨麻；白读读xún，如荨麻疹。

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[Wikisource][2] has 普通话异读词审音表（修订稿）[2016] posted in full plain-text. Along with the [original 1985 version][3].


  [1]: http://blog.sina.com.cn/s/blog_4c0596f60102yg1x.html
  [2]: https://zh.wikisource.org/zh-hans/%E6%99%AE%E9%80%9A%E8%AF%9D%E5%BC%82%E8%AF%BB%E8%AF%8D%E5%AE%A1%E9%9F%B3%E8%A1%A8%EF%BC%88%E4%BF%AE%E8%AE%A2%E7%A8%BF%EF%BC%89
  [3]: https://zh.wikisource.org/wiki/%E6%99%AE%E9%80%9A%E8%AF%9D%E5%BC%82%E8%AF%BB%E8%AF%8D%E5%AE%A1%E9%9F%B3%E8%A1%A8