@ufanautas welcome to the forum!
Did you make the config change before or after you started ingesting data? Existing indices won't be affected, so you would have to perform a manual rollover on your APM index aliases (apm-7.15.1-transaction, apm-7.15.1-span, ...). See Rollover API | Elasticsearch Guide [7.16] | Elastic for how to do that.