The Electrum development team released a minor upgrade a couple of days ago.
I am not sure this upgrade make an effect on the bump fee transactions, as I remembered a couple of weeks ago when used 4.4.3, I still can bump the fee even if I max the transaction. but today, I can't bump the fee with the same situation, I don't know what happens, or they change the default setting. Thanks
* I am stuck on 9 sat/vb fee last a couple of days, but I'm not in a hurry.