FPDI can't add image after GhostScript?

Question

PHP

There is a PDF file. It is necessary to apply a stamp there (normal png). For this I use FPDI. Everything is fine, the stamp is applied.

But as soon as you use pdf version > 1.4 (for example, they are given by Google Docs when exporting a file), FPDI refuses to work with it. To correct the situation, I use GS, I convert to 1.4. The conversion goes well, the file opens, everything is displayed correctly.

But one has only to give this converted FPDI file, it does not apply a picture there. No mistakes. Simply silently spits out that file without changes.

Why can there be such a situation?