With RStudio Version 1.2.1335, Build 1379 (f1ac3452), under Windows 10, in a fresh session, and using R 3.6.0 I find that
help.search("log") produces empty results even though the same command produces many hits when entered in the R GUI. As well, this command works fine in RStudio with R 3.5.3.
My session info:
R version 3.6.0 (2019-04-26) Platform: x86_64-w64-mingw32/x64 (64-bit) Running under: Windows 10 x64 (build 17763) Matrix products: default locale:  LC_COLLATE=English_United States.1252 LC_CTYPE=English_United States.1252  LC_MONETARY=English_United States.1252 LC_NUMERIC=C  LC_TIME=English_United States.1252 attached base packages:  stats graphics grDevices utils datasets methods base loaded via a namespace (and not attached):  compiler_3.6.0 tools_3.6.0