It looks as though you have a very weird assortment of Notation Options set. Probably:
Beam Grouping > Simple Time Sigs Without a Half-bar … Join Beam at Tuplet
Quarter Note Denominator Time Sigs With Half-bars … Beam over rests
Beams and Rests … Use Stemlets
This first beat is easy enough to fix:
Select the three triplets. Edit > Beaming > Beam Together.
That should give you:
I can’t see a way of splitting the third and fourth beats without changing the Notation Options, though (and that’d be the option to beam over rests).
The underlying mechanics, as I understand it, are that in order for beaming commands to apply to rests, those rests have to be made explicit, which is the equivalent of forcing their duration. Percussion kits are effectively made up of a bunch of individual staves - one for each instrument - and then the notes are condensed down to a single staff (or a grid, as in your example). The rests are always implicit - they’re just visual padding - and as things stand it’s impossible for them to take on properties or react to beaming commands.
This has been addressed in Dorico for iPad, so the chances are the same fix will come to desktop Dorico whenever Dorico 4 is released.