I post this so that a reference for the future is available.
Using Okular v 25.12.1 (flatpak) on Linux Mint 22.1
turned off the option “insert text” on the annotation bar.
double click on the text I inserted previously
(in my case) it’s open the “pop-up note” installed on my system. Here I can edit (and save) the text.
It might be possible that the attached editor might be a different one on your system.