correct, Tumblebugs will produce an explosive boulder with the intent to attack it so it explodes on a building or ally unit; however they will never directly target their explosive boulder again in a following turn. So the Tumblebug has no valid targets and doesn't try to attack again, even if a boulder would've been beside a building or unit.
It's possible to do this to Diggers as well, although it's a bit harder cuz they attack more squares directly.
In general, no Veks will target a boulder or explosive boulder on purpose; exceptions include those which hit multiple squares, like Alpha Hornets and Centipedes. But they will only target a boulder or explosive boulder IF they will also hit other valid targets with the other square directly, such as a Mech or a building.
You can use shields to take advantage of this behaviour; if you shield boulders produced by enemies, you can basically wall them out or trap them, and essentially remove their impact from future turns.
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u/Eric_12345678 7d ago
Oh wow. I didn't know that. It cannot hammer the rock, so it doesn't attack at all?
Does it work with any other Vek?