Hide/Show Preview shortcut fails after using custom keybindings

When composing a new message, it is not always possible to immediately toggle the Preview pane via its assigned keyboard shortcut. Using custom keybindings appears to trigger this behavior.

My system: MailMate/5164 MacBookPro11,3/x86_64/8/10.11.1

Steps

1. Ensure your MailMate is set so your default Markup option is Plain text and the Preview pane is hidden

2. Quit MailMate (this is important as it works the first time you try it)

3. Launch MailMate

4. ⌘N to open a new compose window

5. Press ^⌥⌘P to toggle the preview without changing the focus from the To: field

6. Press ^⌘M and use the arrow keys to select Markdown as your Markup type

7. Type an email message to yourself; make sure the Preview pane updates at least once while you are typing the message

8. Send the message using a custom keybinding. (I have "@\U000D" = "send:";)

9. Wait until the message arrives in your MailMate inbox folder

10. Use a different custom keybinding. (I have used a chain like ("makeFirstResponder:", "mainOutline", "selectFirstMessageRow:") but also "deleteMessage:")

11. ⌘N to open a new compose window

12. Press ^⌥⌘P to toggle the preview without changing the focus from the To: field

Expected Results

The preview pane is toggled both before and after using the custom keybindings.

Actual Results

The preview pane is only toggled before using the custom keybinding, but usually you can "recover" the ability to toggle the preview without restarting MailMate.

This bug has been driving me nuts but I haven't had repro steps... until now. I think there's something about doing the above entirely with the keyboard. When I attempted to reproduce it by clicking on the "Send" menu option, I was not able.

Here is my full custom keybindings file:

{
    "g" = {
        "a" = ("goToMailbox:", "C4B7DB3E-74DC-4E8C-8B18-C7F7C69620A1", "makeFirstResponder:", "mainOutline");
        "e" = ("makeFirstResponder:", "messageView");
        "g" = ("makeFirstResponder:", "mainOutline", "selectFirstMessageRow:");
        "i" = ("goToMailbox:", "INBOX", "makeFirstResponder:", "mainOutline");
        "m" = "goToMailbox:";
        "u" = ("goToMailbox:", "EA2F402D-416F-41F6-83C2-F4A3923EB706", "makeFirstResponder:", "mainOutline");
    };

    "G" = ("makeFirstResponder:", "mainOutline", "selectLastMessageRow:");

    "n" = "newIMAPMailbox:";
    "s" = "moveToMailbox:";
    
    "j" = "selectNextMessageRow:";
    "k" = "selectPreviousMessageRow:";
    "^h" = ("makeFirstResponder:", "mailboxesOutline", "goToMailbox:", "INBOX", "selectPreviousMailboxRow:", "selectNextMailboxRow:");
    "^l" = ("makeFirstResponder:", "mainOutline");

    "^@e" = "toggleEncryption:";
    "^@E" = "toggleSigning:";
    "@\U000D" = "send:";
    "$\UF728" = "deleteMessage:";
    "$\U0008" = "deleteMessage:";
    "@k" = "mailboxSearch:";
    "/" = "mailboxSearch:";
}