Joy-Con disconnection issue in handheld mode

The right Joy-Con of the Nintendo Switch repeatedly disconnects and reconnects after a few seconds when in handheld mode.

While all other functions boot and work normally, the right Joy-Con keeps cycling between disconnecting and reconnecting. Once connected, it stays active for a few seconds before dropping out again. The details are as follows:

  • The same issue occurs even when connecting another right Joy-Con.

  • The rails and rail connectors on the main console have been inspected and confirmed to be in good condition.

  • There are no issues at all when using the controller wirelessly.

  • In handheld mode, the connection drops briefly and reconnects randomly; it stays connected for anywhere from a few seconds to tens of seconds before dropping again.

  • Several components near the connector were replaced using parts from a donor board, but the symptoms remain the same.

  • The cooling fan is also working normally.

Which specific area should I focus on for further inspection?