I also encountered the same problem. I am trying to upload a presentation with a table and the black border/gridlines disappear when the file is opened in OnlyOffice. The borders of some merged cells was disappeared. Some tables are normal, the borders of some tables disappear, and the borders of some tables become other colors. I am a new user and can only provide attachments in this way .
I hope you can provide an answer.
• Version: 7.4.1 (build:36)table_11.pptx (3.5 MB)