Thanks for finding out the context of the bug.

The fish could either be removed upon placement or block foundation placement. In alpha 22 it is deleted.
It's indeed this commit and not the D21 one, since we incorrectly translated the line // If obstructing fauna is gaia or our own but doesn't have UnitAI, just destroy it into corpses.
(template_unit.xml states that BlockFoundation = false and BlockConstruction = true, so the template could be considered wrong before and the code was wrong before too and the bugs cancelled each other out.)

To mimic a22, we can set DeleteFoundationUponConstruction true for fish.
Do we have any other unit obstruction that doesnt have unitAI?

Index: binaries/data/mods/public/simulation/templates/template_unit_fauna_fish.xml
===================================================================
--- binaries/data/mods/public/simulation/templates/template_unit_fauna_fish.xml	(revision 21808)
+++ binaries/data/mods/public/simulation/templates/template_unit_fauna_fish.xml	(working copy)
@@ -13,6 +13,7 @@
   <Obstruction>
     <BlockMovement>false</BlockMovement>
     <BlockPathfinding>false</BlockPathfinding>
+    <DeleteUponConstruction>true</DeleteUponConstruction>
   </Obstruction>
   <Position>
     <Altitude>-2.0</Altitude>

Yep, i've just tested the patch and it works as expected. I don't have in mind any other template which could have that problems.
By the way, i've seen that problem (and used also for checking the patch) on wild lake random map. There are so much fishes on that map that it's ideal for testing that patch, but i don't think that is what was wanted. Maybe something to fix in that map for a24?