iphone readycloud image sync crunches index

I have an issues with the iphone image sync feature.

Starting from april 2020 onward, the synced images on my Readnas have a strange naming. Until then the images were synced with a naming like "img_1234.jpg", where the "1234"  number was evolving. So far so good.

From then on, the naming scheme was somehow crunched and changed to "IMG_2147483647 (123)", where only the index in the brackets (123) is evolving.

funnily videos are synced with the prior indexing pattern.

When I sent an image via email from my iphone, the index is conform to the prior indexing pattern, although synced to the readynas with the crunched pattern.

I allready reset on my iphone the cache of the readycloud-app, without effect.

Any hints how to restore the prior indexing pattern for the image-sync..?