If you're not using 20XX, you should use 20XX to test this. Also I'm not 100% postivie about how/when you hit the AI but it looks like you're hitting it out of Firefox and when you UPB is interrupted you're unable to meteor cancel again for 40 frames. Same goes for jumping.
There was a thread on Reddit a while back about the specifics of meteor cancelling and how it works/how long you can be "locked out" of it.
Sadly I'm at work procrastinating way too much and don't have the time to dig around for it but if I can find it later today I'll post it here for you. I don't think the late hit of dair isn't a meteor, but more likely the CPU being unable to meteor cancel. Again I don't know if you're using 20xx or vanilla because you didn't specify but I hope that this helped a little bit
Edit: I hate my job
https://www.reddit.com/r/smashbros/comments/4ajyf1/i_looked_into_the_mechanics_of_meteor_canceling/
This is pretty long and really goes into how it works. I'm not going to sit here and claim that my idea is definitely what's going on, but based on the screenshot and your description this is my best guess.