Has this happened to anyone else?

I originally had 4 student loans (2 Subsidized, 2 Unsubsidized) and I decided to start paying off some of my loans, especially the unsubsidized since interest accrues right away. There are a couple payment methods such as Pay Online, Pay By Mail, etc. I decided to do the Pay By Mail route. I sent a check that would've covered the full amount of one of my unsubsidized loans. I also attached a note saying which specific loan I wanted it to apply. Instead, they distributed the check I sent into 3 different loans (idk how they decided that) and payed off one of my subsidized loans instead. Now, I don't know if this was done on purpose, but it feels like they completely disregarded where I wanted the money to go because they want me (and I'm sure others) to keep getting more interest because it's better for them. Is this normal?