availability?

So I've been a shift lead at Walgreens for over a year now and when I originally was hired, I told my store manager I had open availability. I rarely close and mainly do openings, the occasional mid shift if needed. I've closed before in the past but I recently put in an availability change that I can no longer close and by recently, I mean like over two weeks ago. I put my availability 5:45am to 10pm which I thought would be fine considering I haven't closed for over a month now. So when I checked the schedule today and saw that I was put on a closing shift, I texted my (new) assistant manager and asked. She told me (for the first time since I've worked there) that shift leaders are required to have open availability and she denied my availability change request. I was not told that when I first got hired. Though I understand why, I think it's crazy that I'm the main one opening every Saturday and Sunday for the past year accommodating other people's availability. I come in when people call out (which happens a lot at our store) and I've never called out once since I've started. For over a year now. I'm not trying to toot my horn or beg for compliments. I just think it's crazy that I bend over backwards for this store and me saying "hey I can't work the last two hours out of the 18 hours the store is open" isn't really too much to ask especially when there are four other shift leads. Am I overthinking it?