Using C-c C-f C-<x> in AucTeX randomly inserts font commands in old TeX style ({\sc xxx}) or modern one (\tesxtsc{xxx})

Question

I'm using AucTeX for years now...

I'm using emacs "26.3 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.11)" and AucTeX 12.2.0 (installed from MELPA) on XUbuntu 19.10.

According to doc[1] (and years of usage...), hitting C-c C-f C-<x> should insert (\text<xx>{yyy}). e.g. \textsc{yyy}

However, on my machine, AucTeX randomly inserts font commands in old TeX style ({\xx yyy}) or modern one (\text<xx>{yyy}).

I have absolutely no clue why the behaviour is not consistent accross all .tex files, nor why it does not respect the documentation (always using modern syntax)....

PLEASE HELP!!!! This randomness is REALLY getting on my nerves!!! How can I get back a consistent behaviour???

[1] https://www.gnu.org/software/auctex/manual/auctex/Font-Specifiers.html