Images disappeared in gallery when using a hack to number them

I’m using this code https://pastebin.com/n1mfN3NX to display numbers above images in any gallery of my website.
It work perfectly when adding new images but when used with old post, occasionally will delete the whole gallery and leave only the first image.

This is an example of the gallery working perfectly:
https://bottegapazza.com/wunderkammer/?p=10258

This other is an examply of an old post in wich all images except the first have disappeared from front end(but still visible in the back end):
https://bottegapazza.com/wunderkammer/?p=10258

I dont know where to start to find a solution.
Any help is more then helpfull! Thank you.

Ps. I have an idea about the problem but I’m not sure about it.