I've already discovered that issue in a TYPO3 6.2.x Version and now having the same problem in TYPO 7.6.x.
It's always the first image of a row not resizing, no matter how images added to an element or how many rows configured. TYPO3 is somehow obviously trying to do something with the image, as the source is "/fileadmin/processed/.." - but the image size is the original size. All other images in the row have the wanted and configured size.
I've found a 3 year old entry in the TYPO3 Board with the same problem, but no solution: https://forum.typo3.org/index.php/t/195320/content-type-quot-images-quot-erstes-bild-wird-nicht-verkleinert So it looks like i am not paranoid at all. : )
Anyone else discovered that issue? Is there a solution for that?