This is a known issue in v4.0 (Vivado 2017.4) and earlier versions of the software driver. It will be fixed in the HDMI TX Subsystem software driver v5.0, in Vivado 2018.1.
This problem is due to the HDMI TX Subsystem only setting the minimum set of AVI InfoFrame parameters. Some HDMI Sinks cannot detect the Pixel Repetition information.
Some sinks are expecting additional AVI InfoFrame fields to be set so that they can properly detect the incoming video.
Specifically for 576i, some sources are expecting the following fields to be set:
Vivado 2017.4 and early versions - You can add the above additional fields, and send custom AVI InfoFrames.
For more information on how to send custom AVI InfoFrames, see (Xilinx Answer 68780).
Vivado 2018.1 and later versions - This issue is resolved.