Whiteboard sometimes does not display all graphics on uploaded pdf-slides

[SimonLoebert]

Describe the bug
In PDF documents that are uploaded to BBB and displayed on the whiteboard, not all graphics are displayed in some cases.

This problem does not occur for all users, that are in the conference, at the same time, but only for some. I have not yet been able to determine which requirements must be met by these users (e.g. specific OS or web browser).

Slides on which graphics are to be displayed in different layers (e.g. image in the background and individual smaller graphics in the foreground) are particularly affected (see screenshots).

To Reproduce
I wasn't able to reproduce the bug on my system (Mac OS + Firefox and Android + Chrome) but I have received several screenshots from users that clearly show the problem.

On my PC I have observed that these are sometimes displayed with a delay of a few fractions of a second, but I have not yet been able to prove that I am experiencing this problem.

Expected behavior
Display of all graphics (as in the PDF viewer)

Actual behavior
Some graphics are missing in the whiteboard for some users

Screenshots
Desired display:

Actual display for some users:

BBB version:
The problem occurs on different servers with the versions 2.7.1 and 2.7.6. BBB was installed via the bbb-install.sh script on Ubuntu 20.04.

Desktop:

• OS: Windows 10
• Browser Chrome and Firefox
• Version unknown (users state that they are using a current version)

Additional context
A reinstall of the server unfortunately didn't solve the problem.

Please tell me, if you need any logs. Thanks in advance!