first, my report didn't claim a problem with valgrind, but with TSAN (thread sanitizer). Similar, but not the same. My report ALSO claimed that you can switch to SQLITE_OPEN_FULLMUTEX to workaround the problem. That - I believe - was correct for TSAN, but I've since tested with valgrind, and I get the same error report from valgrind (even using SQLITE_OPEN_FULLMUTEX). Reverting to 3.36.0 fixes both the TSAN and Valgrind issues. I am in the process now of gathering a more full report on a generic DEBUG build - both WITH TSAN and with valgrind. I should be able to upload these reproductions later today.