photoshop change my resolution when exporting

Allow me to up this message. 
I have encounter the exact same problem. I like to work with artboard to have the whole set of variants of my document, and the only solution I have is export... so for me it a real issue and must be considere as a bug. I hope it will be corrected soon.

Let's just wrap this up while we're at it, because there's a basic misunderstanding here that is admittedly very easy to make:

In most other software, the normal method for producing a finished file is "export". That's what you look for when the work is done. Not so in Photoshop. Here you use "Save As" to produce a new, finished "master" file. Save As is what you normally do. By master file I mean a full, complete new file, one fit for storing/archive, further processing later, sending out to others - and printing.

In Photoshop, Export is a very specialized function for a specialized purpose - namely, to optimize a file for web and mobile devices. A big part of that is reducing file size in every possible way, including stripping out everything not strictly necessary. That's why the "pixels per inch" metadata are removed. It's not needed for the intended purpose, screen and web.

That was pretty clear back when Save For Web was the promoted tool for this. Now that's on the way out, they very unfortunately named its successor Export, which only leads to all kinds of confusion like here.

As for why ppi is irrelevant for web, it's because a screen already has a resolution: the screen pixel grid. On paper, there is no such grid, so one has to be invented, and that's the "pixels per inch" number. It defines the pixel density on paper, and thus how big the image will print. Size and ppi are inverse: as one goes up, the other goes down.

Use save as rather than export or save for web, if you want to print your images. Export and save for web, are just for web images.

This is nothing new, you've just noticed it now.

Export (and Save For Web) strips resolution metadata altogether. An exported file doesn't have a ppi value at all, neither 72 nor 300. It is ppi-less because it's not needed for screen use, only for print.

The ppi number you see when you reopen the file is just whatever default value the opening application assigns. In some apps it's 72, in some 96.