Java Library for JasperReports - Random font size and styling issue in massive report generation

I am using JasperReport in my recent Java project for PDF generation purpose. It's working perfectly most of the time, but somehow in a few of the output PDFs(maybe 1 out of 5000), we saw that some of the contents are having weird font size and style (sometime smaller, sometime missing and sometime losing all the added text styles).

We've built multiple services in parallel for the huge amount of PDFs creation, and will generate roughly 150 PDFs per minute in one single service.

Also, we've already checked the output .jrxml files and the data parameters in debug mode. The abnormal PDFs look just the same with the normal one. The behaviors seems completely random and non-sense to me.

I've spent lots of time on it but still no lucks... Does anyone face the same issues with jasper reports or have any clues, and please let me know if there's anything unclear, I will be very appreciated for any help, thanks so much!!!!!