I don't know if it's common or not
It made my life much easier to navigate the transition to 1 nap, because I could just keep pushing that morning A time, let her sleep as long as she wanted, and still get a catnap in before bedtime. She started transitioning to 1 nap around 10 mos, finished it around 14 mos. And even now (almost 18 mos), her first A time is still longer - about 5 hrs (sometimes 5.5), but between nap and bedtime is usually only 4, and sometimes less (depending on how long she naps). It's what works for Abby, so it's normal for her
If that's what works for your bub, enjoy it - I found it made the switch to 1 nap so much easier to figure out!!