Palmtrees do not block line of sight, but their collision volumes can block units from fireing. My guess(have not watched the replay) is that the palm tree has either a sphere or thin line collision, and the recluse hides behind that hitbox. Since the stinger shoots a straight lazer, the tree blocks the path. if the recluse can shoot the stinger, some units take into account curved/arced fireing lines, so a recluse could shoot over or something of that order. map features and unit wrecks have very noticeable effects like this for example in schorcher battles, where schorchers cannot shoot through schorcher wrecks, as well on maps like andosnia where the small palmtree clusters in the middle by the river can really mess with attacks.

Ok thank you. The palm trees have a sphere, and as Steel_Blue says thats probably preventing Stinger to fire (See this Steam screenshot)

Looks a bit weird. Is this intended?

IMO this makes no sense. No tree should prevent a unit to shoot. It was frustrating, I built that Stinger precisely for killing Recluses.

You can attack ground to force a unit to fire without avoiding trees and wrecks.