It's not a bug, but a feature.
All created HD Accounts are based on the same seed.
These are NOT two hd accounts based on the same seed.
They are two completely independent hd accounts, created from two different seeds.
Mycelium is able to do that. Go to Add New Account --> Advanced. You can scan an HD account that's completely unrelated to anything you have on the phone already.
I didn't create the second hd account from the first one. The two accounts have different seeds.
The second account
in this case is not a sub-account of the first one. It is a
new account with a DIFFERENT SEED.
Mycelium can run two (or more) wallets with different seeds, but only exports the seed of one of them.
That's a bug, and there's no use mincing words about it.
And just for the record, I'm not saying that the ability to run two accounts with different seeds is a bug.
The bug is mycelium's inability to export more than one seed.