readxl currently has a ... memo, I'll call it, from their clang-UBSAN checks. readxl still passes R CMD check and our own tests cleanly but this line gets flagged:

Here's the detailed report:

https://www.stats.ox.ac.uk/pub/bdr/memtests/clang-UBSAN/readxl/tests/testthat.Rout

I suspect that this can be seen as a false positive, in light of external knowledge of the xls spec and memory layout. I'm trying to figure that out for myself, but would be happy for any wisdom or wording you have to share.