I've come accross this limitation or bug and just wanted to report it. It seems as if yad stops parsin stdin data in forms in some cases.
I have tested the form with YAD and zenity. Zenity seems to parse the data just fine, while YAD only parses it if there are less than @27 rows in this case. For simplicity, I'm attaching the full script in 3 samples:
Sample1 - YAD throws error and doesn't create the dialog. Error: Unable to parse command line: Invalid byte sequence in conversion input
Sample 2 - Same bash script replacing YAD with Zenity: works.
Sample 3 - Using YAD with lesser rows than in Sample 1 : works.