Big uncommitted segments to disk reported by GET /myIndex/_segments API (ES version 6.3)

Hello!
WE are using ElasticSearch version 6.3. We have one situation were the GET /myIndex/_segments API reported some big size files that are uncommitted to disk (the committed field of the mentioned API is false, see the attached picture).

Another aspect, is that we also have in these segments a huge number of updated files (we know they are updated and not deleted).

My questions are:

  1. What can cause that these segments do not be committed to disk?
  2. How this situation can impact for example search performance?
  3. What can prevent merging of segments?
  4. After the merging, the deleted number of documents should be decrease, isn't it?

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