It seems that color is not reset to default value after use.
public static class SampleCode {
public static void main(String[] args)

{ log.trace("test"); log.debug("test"); log.info("test"); log.warn("test"); log.error("test"); }

}

logback.groovy:
appender("STDOUT", ConsoleAppender) {
withJansi = true
encoder(PatternLayoutEncoder) {
pattern = "%d

{yyyyMMdd HH:mm:ss}

%blue(%mdc

{contener}

) %thread %highlight(%-5level) %msg %n"
}
}
root(DEBUG, ["STDOUT"])

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In ForegroundCompositeConverterBase (line 36) color is reset by appending:
sb.append(SET_DEFAULT_COLOR);// ESC_START+DEFAULT_FG+ESC_END;

ANSIConstants.DEFAULT_FG has value: public final static String DEFAULT_FG = "39";

After searching in jansi sources, I've found that color reset is invoked by inserting \033[0m
(and it works for me too )