Very recently, this has been intensively analysed and identified as being caused by specific behaviour of unbound, see Two Pi-hole instances, One with "failed to send..." - #44 by DL6ER.
There is currently nothing that Pi-hole could do to improve this.
The condition has been reported to unbound, where some potential measures are currently explored (e.g. increasing unbound's incoming-number-tcp).
You may consider to contribute your observation over at failed to send TCP(read_write) packet (Connection prematurely closed by remote server) · Issue #1237 · NLnetLabs/unbound · GitHub.