See the issue in this thread https://forums.autodesk.com/t5/revit-structure-forum/rvt20-modify-plate-inside-custom-connection/td-...
If a custom connection is created which contains plates and or bolts these components can't be edited (parameters changed, boundary edited, etc) when in "Edit" mode of the custom connection - they can only be removed which then deletes them from the model. The only workaround I have found is to create replacement plates/bolts in place outside of the connection and then remove (delete) the old plates/bolts and add the new ones when in "edit" mode of the connection.
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