读取时文件被截断

I am writing some json results in files in PHP on shared hosting (fwrite).

Then I read those files to extract json results (file_get_contents).

It happens some times (maybe one out of more than one thousand) that when I read this file it appears truncated: I can only read a multiple of the first 32768 bytes of the file.

I added some code to copy/paste the file I am reading in case the json string is not valid, and I then get 2 different files: the original one was correctly written as it contains a valid json string and the copied one contains only the beginning of the original one and has a size of x*32768 bytes.

Would you have any idea of what could be the problem and how to solve this? (I don't know how to investigate further)

Thank you