I believe we can close this bug report. After quite a lot of debugging, it looks like our environment had built a broken version of the spec-alpha-clojure package. After rebuilding spec-alpha-clojure again, in a build environment containing libcore-specs-alpha-clojure 0.4.74-1, clojure 1.12.0-1, and spec-alpha-clojure 0.5.238-1, the issue resolved itself. I'm not sure which build-depends versions were used to produce the broken spec-alpha-clojure in the original bug report, but at least one of these three was likely an older version at that time.
Sorry for the hassle.
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