If a Throwable type is passed to match the last {} argument, it is ignored, and the formatted log message will write {} instead.

package com.test;

import ch.qos.logback.core.Appender;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.slf4j.event.Level;

public class Test {
  static final Logger log = LoggerFactory.getLogger(Test.class);

  public static void main(String[] args) {
    log.info("SLF4J Version: {}", Logger.class.getPackage().getImplementationVersion());
    log.info("Logback Version: {}", Appender.class.getPackage().getImplementationVersion());

    Exception ex = new CustomException("Some message");
    log.error("Exception Message: {}", ex, ex);

    log.makeLoggingEventBuilder(Level.WARN)
        .setMessage("Exception Message: {}")
        .addArgument(ex)
        .setCause(ex)
        .log();
  }

  static class CustomException extends Exception {
    public CustomException(String msg) {
      super(msg);
    }

    @Override
    public String toString() {
      return "Custom Message";
    }
  }
}

Expected output:

[main] INFO com.test.Test – SLF4J Version: 2.0.15
[main] INFO com.test.Test – Logback Version: 1.5.11
[main] ERROR com.test.Test – Exception Message: Custom Message
com.test.Test$CustomException: Some message
at com.test.Test.main(Test.java:15)
[main] WARN com.test.Test – Exception Message: Custom Message
com.test.Test$CustomException: Some message
at com.test.Test.main(Test.java:15)

Actual output:

[main] INFO com.test.Test – SLF4J Version: 2.0.15
[main] INFO com.test.Test – Logback Version: 1.5.11
[main] ERROR com.test.Test – Exception Message: {}
com.test.Test$CustomException: Some message
at com.test.Test.main(Test.java:15)
[main] WARN com.test.Test – Exception Message: {}
com.test.Test$CustomException: Some message
at com.test.Test.main(Test.java:15)

Workaround:
Explicitly calling .toString() on the argument (log.error("Exception Message: {}", ex.toString(), ex)), or adding a bogus extra "" argument (log.error("Exception Message: {}", ex, "", ex)) works as a work around, but causes static analysis tools, including the one in IntelliJ, to warn about it.