Solution found.
I set up a simple test case on a desktop machine (Ubuntu, with xampp) and my test account at the shared hosting company (www.icdsoft.com - they are wonderful).
The local version (xampp) was OK, the simple test case at the hosting company showed the same fault - i.e. above 100 images the metadata was not saved.
I raised a ticket with ICDsoft, which was immediately taken. After 38 minutes of investigation (an unusually long time, but this was a relatively hard query) they had the answer. Here it is:
Hello,
We checked this. The problem was caused by the default limit (1000) of the max_input_vars variable. We created a global php.ini file for your account and added the following line to it:
max_input_vars = 2000
Thus, we increased the limit to 2000. You can modify this line according to your needs.
You can edit the global php.ini file for your account via the hosting Control Panel (https://xx.xxx.xxxx..com) -> the "System Information" section -> the PHP Settings (edit) link -> the Advanced Mode link at the left-hand side. More details are available at:
http://www.suresupport.com/faq.php/71/468
The "max_input_vars" setting was introduced with PHP 5.3.9. Servers running versions prior to 5.3.9 would not experience this problem.
Let us know if we can assist you further. Thank you.
Best regards,
Support