Threading macro with update and when produces incorrect results

Question

Environment:
- ClojureCLR version: 1.12.2
- .NET version: 10
- OS: MacOS 26.1

Description:
The pattern (-> map (update :key #(when % (fn %))))
fails in ClojureCLR but works correctly in Clojure JVM. When chaining multiple updates in a threading macro where the anonymous function contains when, the result is incorrect or causes runtime errors.

Expected Behavior:
Should work identically to Clojure JVM - update with a function that
returns nil should associate nil to the key.
Actual Behavior:
[Crashes/Wrong values/500 errors in web handlers]
Workaround:
Use explicit conditionals instead:

 (let [value (when (:key map) (fn (:key map)))]
    (if value (assoc map :key value) map))'

Comments

I tried the following in Clojure 1.12.2 (Windows) and 1.12.3-alpha4 (Windows and Linux):

```
(def p1 {:name "James" :age 26})
(def p2 {:name "Fred" :age nil})
(def p3 {:name "Fred"})

(-> p1 (update :age #(when % (inc %))))   ; => {:name "James", :age 27}
(-> p2 (update :age #(when % (inc %))))   ; => {:name "Fred", :age nil}
(-> p3 (update :age #(when % (inc %))))   ; => {:name "Fred", :age nil}
```

I thought the `p2` and `p3` examples matched the description given and would fail.  They do not.  Could you provide a more specific example that illustrates the problem?   Your description says the error occurs in web handlers?  Does this only occur in that context?