On Android, when using "Export as..." option (2048px, 100% JPG, sRGB, no output sharpening) when saving a single photo that is currently open (Info, Edit view, etc) the quality is better than when selecting multiple photos from an album view.
The two "_bulk" photos were exported by selecting just those two and using "Export as..." option with the same settings. First of all, you can see that the "_bulk" photos are smaller, additionally:
On the 1-st photo, with the cat, the bulk one looks more blurry, you can see some JPG artifacts (e.x. near cat's whiskers) and image is overall a bit darker.
On the 2-nd photo, with birds, the compression is harder to notice, but on the other hand it looks like chromatic aberration removal option was not applied, despite it being enabled.
I exported the cat photo through the website too, and it has that better quality, so it seems that only exporting multiple photos on mobile has an issue.
I'm using Android 11 on Sony Xperia 1 II and Lightroom v6.4.0.
I did a bit more testing and it seems that the issue is - the photo is exported with a lower quality from anywhere but the Edit view. Attached is the same photo exported with the same settings from 1 - album view by selecting just this single one, 2 - from Info view after opening the photo and 3 - from Edit view. You can see the chromatic aberration on the tree and the blurriness of leaves on the trees in the center on 1-st and 2-nd file. The third one looks the same as it does when viewing the photo in Lightroom (no chromatic aberration, leaves are not blurred).
I made a screen recording of the process. Pay attention to the edges of the tree when viewed in the app, on the first export, and the second one. Also when you pause on the file browser, you can see that the sizes of those two files differ.
Export a group of images (containing the first image)
Create an Android Log File
Open Settings Panel, long press on the top left “Lr” icon. This will enable a “Send Feedback” button on the settings panel.
Now perform the usual tasks in LrA for few minutes and then please reopen the Settings Panel and tap on “Send Feedback” from settings panel. This will open up your email client after few moments and attach the logs.
Copy yourself on the logs just in case.
Post back here and let me know you've completed the process.