That field is custom, it doesn't come with JIRA, meaning an admin must have created it. There must be something else going on with that, since fields changing value on their own is not a standard behavior.
If it changed on a transition, I would have suspected a workflow post-function. But a change after an edit, that sounds like something that might have been done with the Behavior plugin (now part of ScriptRunner). Check if there's a behavior set for your issue type in the add-on part of the administration section. If that's not the case, something strange must be going on, and you'll need to supply more info, and potentially open a support request with Atlassian.
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