One issue: Sometimes I want to start with some letter, but the game only marks the second letter, rendering the selection incorrect. Might be due to my computer being busy with some simulations though.
I think what's happening is you're typing in a few letters of one word, then moving on to the next word with a new letter that's not adjacent to the previous ones. Typing a letter that's not adjacent causes the word to fail, and that letter won't be part of the next word. So if we had a 3x3 board
A B C D E F G H I
And I typed ABC GHI, it would only count HI since G was part of the failing word ABCG.
This is how it's supposed to work, but it can get a little trippy some times. If what I explained is not what's happening, and it's just not picking up the first letter in a normal case, let me know!