Thanks for using the Apple Support Communities. It looks like no one has responded yet to your question. If you could provide the following information, it may help the community respond:
1. What model iPhone is this happening on, and what version of iOS is it running? (Find the software version on your iPhone, iPad, or iPod - Apple Support)
2. What is the exact customization you are doing, and what customization is being deleted/changed on the other contact when you do so? Is this only affecting certain specific contacts, or is it contacts in general. If you create a brand new test contact and try to customize it, does the same thing happen?
3. What is your current contact configuration? Are you syncing contacts with any other source?
4. How long has this been going on? Was it working successfully previously? If so, has anything changed recently (new contact source or sync, new app or OS update, etc)?
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Question: When I make changes to one contact it deletes my custimizations to another contact