Encountered difference in X-ray analysis

I think you'll have to also check the package markings from the outside as well. According to the legend, it is possible that you received products from different batches that came from different plants:

Differences in Xray micrographs don't necessarily mean that a part is non-conforming - it's not a guarantee that every part will look the same as many semiconductor manufacturers may have the same part being manufactured by different plants in different countries, sometimes with slightly different packaging. Perhaps a better indication would be to examine the dies themselves if you can decapsulate them. From a brief glance at your images, I estimate both dies have about the same size and connections, which suggests to me they're internally probably identical and this is just a difference in package assembly.

Nevertheless, if it's been purchased from a reputable electronics source such as Avnet, then you should be able to contact them directly and request traceability data to know where/when it was sourced.

Finally, it may be worth contacting the manufacturer of the part directly (e.g. onsemi via https://community.onsemi.com/s/ or https://www.onsemi.com/company/about-onsemi/quality-and-reliability/quality-contacts) as they would have more knowledge about their manufacturing process than outsiders such as myself would.

- Gough

For the most accurate answer you need to contact ON semi directly. But on mouser I found 3 product change notices (PCN). Probably the most interesting you is the following one:

But it is still dated to 2021. Maybe you get some older units when you were ordering them for first time? Chech date codes of parts. Also as Gough Lui  mention assembly location and if it is very important to you, then contact ON semi directly.

Thank you Gough. This is very helpful!

Thank you. We will definitely check with Onsemi. 

As pointed out by others the manufacturer is the most knowledgeable source for this kind of dilemma.

The die used for cutting out the frame also looks different.

Or just buy new ones, if this is a critical application, is it even worth using them if there's a shadow of a doubt?

If they are a year old anyway. It's a jellybean part, that will cost under $300 to replace the whole lot.