The CMAP specification (DRIVERS-3218) requires that connection-establishment errors carry the SystemOverloadedError and RetryableError labels so SDAM can preserve the connection pool under server backpressure.
The same spec explicitly excludes several error categories from labelling, including SOCKS5 proxy failures:

"For errors that the driver can distinguish as never occurring due to server overload, such as DNS lookup failures, non-I/O TLS errors, or errors encountered while establishing a connection to a SOCKS5 proxy, the driver MUST NOT add backpressure error labels for these error types."

 References

• https://github.com/mongodb/mongo-java-driver/pull/1900
• CMAP spec section "Backpressure-enabled" — non-overload error carve-out.
• JAVA-5205 — SOCKS5 in async driver.