I think you might want to think about slightly changing font type (or spacing) when using bold parameter (if that is even possible).
It's fine for most numbers but letters are merging for me and it's not easy to read all of them, that can be a problem when someone want's to verify BTC address.
I think I have a solid eyesight, but other people could have even bigger problem reading than me
It is a default php font. I don't know what it is...
This might be Tiny font
https://stackoverflow.com/questions/9842068/what-is-name-of-default-font-used-in-php-gd-library
I don't know the cause of this bug, I don't see anything in logs
But it is probably related to too many requests , it usually happens in a page like this with multiple images in the same page for testing.