Strange negative Numers as errors

[franky4]

I have to make an realy big excel-report. At first I've got java heap space errors an I increased up 20 GB.

-Xms1024m
-Xmx20000m

Windows 10 task manager shows usage up to 18 GB

There is no heap space error anymore but I get different negativ red numbers instead of the java heap space errors (attached pic), depending on the width of the report (here 5000 px).

Even the java preview generates that errors (Preview with Studio 6.16.0 CE).

But csv-file seems to work with 16000 lines an no error. Perhaps it's not finished yet.

Despite the error Excel file with also 16000 lines is created, but I think also not finished.

Any ideas explanations to the negative red numbers, the meaning and how to fix?