Inconsistent opacity / transparency

Here are two images with my watermark applied. Same image with different crops. The watermark should appear as it does in the lighter version.

https://cdn.holypixels.net/wp-content/uploads/2017/03/06200251/church-176059-800×600.jpg
https://cdn.holypixels.net/wp-content/uploads/2017/03/06200251/church-176059-600×600.jpg

In my case, the 600×600 is the medium size in Settings>Media. It seems to be the only size that has the watermark applied with the wrong opacity.