First, because image in base64 won't work and it annoyed me to create a html file for an img src... (or just copy/past in a new tab, but I was only at my second coffee 
)
➤ how to upload image > https://www.gimp-forum.net/misc.php?action=help&hid=11
More seriously, now your question
This is not a "bug" if I can say that.
I did a test, as you can see below I made a transparent area, then did an "Alpha to Selection", thus selecting all the opaque except the transparent inside and got the same error
when you launched the heal selection the second time, maybe your selection got "inverted", making the "Content Sampling Width" off?
If not, please show us your image
)➤ how to upload image > https://www.gimp-forum.net/misc.php?action=help&hid=11
More seriously, now your question
This is not a "bug" if I can say that.
I did a test, as you can see below I made a transparent area, then did an "Alpha to Selection", thus selecting all the opaque except the transparent inside and got the same error
when you launched the heal selection the second time, maybe your selection got "inverted", making the "Content Sampling Width" off?
If not, please show us your image
Patrice