It's saphalitis. A reproductive infection.
Extra protein doesn't keep the heart muscle from being consumed as the heart muscle doesn't ever get consumed. How old and what breed is your hen? What's her main diet?
Reproductive issues are often genetic and then compounded by poor diet.
I'm actually not sure of the original intent of your post, but I have a low grade migraine so I'm not 100% functional.😅
I'm not sure why some stores have some meds and some don't. Again, I'd just stock up and not draw undue attention to that store.
I can still get most mycins here.
If you got it, what are concerned about? Don't look a gift horse in the mouth.
Most stores aren't receiving any new stock but can still sell old stock without issue. Also certain antibiotics (seemingly) don't fall under the new law, such as terramycin, duramycin etc.