Elaine Gould’s Behind Bars states, on page 226:
“For semibreves, position the tremolo beams as if the semibreves were stemmed”
" It is acceptable to move semibreve beams close to noteheads, to save space outside the stave. This should be avoided if the close proximity of beams and notes or ledger lines is visually confusing".
As far as I can tell, Dorico’s defaults do exactly that. The defaults are optimised.
If you want to change them, you have the choice to do so. You can even do it globally if you like: