You never said you checked anything. You never said "WordPress is generating thumbnails properly." You never said "I've disabled all plugins and reverted to default theme, but the issue persists." You never said "I've checked to make sure that 'thumbnails' is checked in the ACF admin for that custom field." In fact, you (again) stated it's not a problem with your theme or other plugins, but you apparently came to this conclusion by just deciding it. Yes, an update caused the issue. Unfortunately for you, because it is not an issue for most users, the issue is in one of your plugins, your theme, or your server setup.
First rule of WordPress debugging - turn off all plugins and revert to default theme. If you can't even do that and come back and say you have with the results, then no one will treat this as a viable bug.
Again, this is not an issue for most of the users of the plugin. This means that no, the problem is not guaranteed to be the plugin. I understand your logic of "I updated this and now it doesn't work, therefore it's this." I also understand that it does not always work like that, and proper troubleshooting is the only way to find out for sure.
So, once again, here is the ONLY WAY YOU WILL GET HELP.
Go into that specific image field's settings (Custom Fields->Custom Fields->Your Field Group-> Your Image Field). Make sure "thumbnails" is selected on "preview size". If that is not the issue, disable ALL plugins and revert to Twenty Eleven theme. If the issue persists, then the problem is with your server setup. If the issue does not persist, there is a conflict with either your theme or another plugin. Once you've distilled that down, you can report back what your system setup is (I will be happy to explain how to get the useful info) or what plugin is causing the conflict.
You can complain all you want, but until it can be reproduced on a vanilla install you're wasting your breath. I cannot reproduce the issue - images properly load the thumbnail preview regardless of location of metabox or size of image.
So yes, you acting entitled and impatient, berating someone who's trying to help you, refusing to do basic debugging or not reporting back clearly that you have tried it, and not gathering the information requested of you is a great reason to be a jerk. After all, you're getting exactly what you gave, mate. Time to reconsider what you're giving.