unexpected pipeline result: *pgconn.PipelineSync

**Describe the bug**
When usign a batch to insert an object I've got following error: `unexpected pipeline result: *pgconn.PipelineSync`

**To Reproduce**
Steps to reproduce the behavior:

I've failed to reproduce

**Expected behavior**
Error message will tell what's the problem is.

**Actual behavior**
Error message that looks like internal pgx issue.

**Version**
- Go: go1.20.4 linux/amd64
- pgx: v5.5.0

**Additional context**
Add any other context about the problem here.

No source file, test, or entry point is identified. Start by investigating the batch insert path with Go 1.20.4 and pgx v5.5.0, focusing on the unexpected *pgconn.PipelineSync result; completion would require a reproducible case and an error message that explains the problem instead of exposing an internal pgx detail.

Written by the indexing model from the issue text.