The sprite handling code hasn't changed between 1.75 and 1.76, but it should be fairly straightforward to troubleshoot this. In almost all cases, it has been because one of the following:
1. Edit > Project Settings > Quality - Texture Quality should be set to Full Res, also on the configs for your target.
2. Select the atlas texture, and there shouldn't be any override, and try setting max size to larger than the texture size (Set it to 4096 and click Apply)
3. Make sure it isn't compressed (Format set to Automatic Truecolor)
4. Try disabling mipmaps (switch texture type to Advanced, untick Generate Mip Maps) - if this happens it means the
5. If you're using tk2dCamera, make sure your viewport isn't squished - the default behaviour with this changed slightly, but shouldn't affect already set up scenes.
Let me know if any of that fixes it. Perhaps some unrelated change in the code has triggered this. If anyone is experiencing anything like this, let me know too - I'll get it sorted for the next release.
@pdform, if you can set up a test case with this happening, I would be most grateful if you could send me a repro case. I should be able to fix this straight away if I can see the issue... (support@unikronsoftware.com).