This seems related to ASYNC-155, a small repro:
This bug affects all version of core.async, but it's getting triggered more now that the patch for ASYNC-138 applies a lexical transformation that makes code look like this, thus hitting this bug more often
Approach: The approach is the same used for ASYNC-155, applied to :let binding
Patch: 0001-ASYNC-187-preserve-type-info-of-locals.patch
Applied for next release.