Lightroom changes the resolution of pictures automatically from 300 to 240

I don't know where you look in LR to see that the resolution is 300, but there is a preference that says what to do when using Edit In PS from within LR, at least when the two versions don't match, and in my LR it's set to 240, so maybe that is where the 240 comes from:

Or if you are Exporting photos to disk there is a place to set the resolution and on my LR it is also 240:

These are screenshots from LR 7.2 so maybe yours will look slightly different but these Preferences have been there for years.

When I work on an image on lightroom and then send it over to photoshop, the resolution is automatically changed from 300 to 240. Does anyone know why it happens and how it can be fixed?

You can specify the format, color space, bit depth, and resolution of the images created by Photo > Edit In using the options in Preferences > External Editing.

Btw changing back the resolution on photoshop doesn't work for this.

In my configuration, when I do Photo > Edit In > Edit In Adobe Photoshop CC 2018, then in Photoshop do Image > Image Size, enter a new resolution, then save the image, the saved image imported back into LR has that new resolution.  (LR 7.2, Photoshop CC 2018 19.1, Macos 10.13.3.)