Support confirmed that this is a bug (SPL-182112) for S2S communication between 8.x nodes. In my case I had issues between SH and INX. The recommendation was to set negotiateProtocolLevel=5 to downgrade the protocol version to 7.3. This can be done in the [tcpout] stanza on the sending node (SH), or in the [splunktcp] stanza on the receiving end (INX).
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