Watermark Problems

I have a very particular problem that needs your full attention.

I have a 40 second video that has to be exact
Buffer size: 1920x1088

Video resolution: 1504x854.

The video contains two joined videos that have different crops, in practice they are two different shots.

The video plays correctly.

I have a problem when inserting the watermark.

In practice by launching the same command to insert the PNG which is 1920x1080

i get different results, and this is really a mystery.

There are times that the png is only seen in the first part of the video that contains the first shot, while when the shot changes, the png is not seen.

And elaborations where relaunching the same EXACT command, the png is correctly inserted in the whole video.

The result is RANDOM, and I’m repeating trying to watermark the same video.

Does anyone know how I can try to fix this?