On mobile, replies to plus aliased addresses need to respond using the plus alias.
Before switching to SimpleLogin, I was using a single "accounts" email address and I was plus aliasing to compartmentalize accounts (email@example.com). In the browser, responding to any emails sent to one of these plus aliased addresses properly responds using the full plus aliased address (firstname.lastname@example.org for example). On mobile, however, this behavior is broken: responses use "email@example.com" and ignore the plus alias.
Came to add this same suggestion just now; thanks for adding it already. I fully agree and it would be nice if the app would allow new messages to be created with an alias as a part of this update as well. Seems pointless otherwise.