Log format is changed after updating promtail to 3.9.1

We recently moved to grafana for centralised logging. To get stacktraces we needed multiline feature which is why we need to update promtail. But after upgrading we are getting weird formatof our logs in Loki:

2022-01-05 23:35:38	
{"log":"\u0009at java.lang.Thread.run(Thread.java:748)\n","stream":"stdout","time":"2022-01-05T18:05:38.684547463Z"}

All other configurations are default. Would be very helpful if someone can help.