I wouldn't say it's an issue but it's a constrain Adobe Express has imposed. Whether it's imposed by Instagram themselves or an oversight from the team, I cannot say.
Did you try to make a feature request for this?
#Scheduler won't let you to post rectangular image on Instagram
native sluice
It seems that nobody ever requested it so for this so I would suggest you add it to the list. You can also share the link in #chat for other people to vote for it...
warped gate
I wouldn't say it's an issue but it's a constrain Adobe Express has imposed. Whe...
I guess this appeared when Instagram introduced its new grid not long time ago. You still can post square images and they work OK with the scheduler, but if you want to adjust to these new rules you have no options. Thank you for the link, I will submit and idea 🙂