I know my 13 DD has lost repect for my IL's due to them constantly being late/ leaving early/ whatever to deal with Other Grandkids. Most any thing we invite them to without Other Grandkids has them showing late or racing to leave early to go pick up the other girls. My DD doesn't want to talk on the phone with them any more and I think would be okay with limited contact at this point.
A bit of a different deal, but still the same idea- grandparents ditching one event for something else.
So yeah, even at 13-14, a grandparents actions can matter, a lot.
Yeah, my grandma did the same thing. The thing is, I wouldn't have really picked up much until I was older, except that my mom constantly brought it up (not necessarily to me, but you hear things when you're 14), which made me feel bad. I really think it would've been a small blip on my radar if I hadn't heard my mom discussing it constantly. I know she didn't mean it, but that was more damaging than my grandma actually going to other events & I kind of resent it a bit. I hate to rehash my own issues, but my mom didn't have a great relationship
with her MIL (not horrible or anything, just not close), and I think that it subconsciously affected how my grandmother prioritized her time. She still did the best she could to be a good grandma, and behave "fairly", but, ultimately, there were little things that weren't really fair. I think my mom cutting her some slack probably would have kept things from continuing on that way. I don't want to project my own experiences on the OP too much, but I just don't think such upset (and cancelling future birthday parties) over such a minor thing is really healthy for the daughter (even if there were a few other minor incidents, without knowing the details, it's really hard to assume the grandparents are so horrible to their granddaughter). That's just my perspective based on my experiences, and something for the OP to consider. It may not apply, but I think it's fair to bring up. I've seen it happen to other families, too. Best of luck to the OP, I know familial relationships
can be difficult.