Looks like an old issue is still valid.
If an row goes over multiple pages, the table header overlaps the text.
As already described here: [bug] overlapping table headers with content from the above row when creating pdf · Issue #15504 · frappe/erpnext (github.com) and traced to the wkhtmltopdf issue: overlapping text when repeating headers/footers in table · Issue #1524 · wkhtmltopdf/wkhtmltopdf (github.com)
I tried all the suggestions on both github issues, unfortunately without success.
Anybody could solve the issue?