Field messages that are initially displayed for variables (or form fields) are removed/hidden after the value in the variable (or field) changes.
Jakarta and onward
The use of g_form.showFieldMsg() (or the legacy g_form.showErrorBox()) to display these field messages are now getting hidden as per the design of the platform starting in Jakarta.
As this is now how it's designed in the platform starting with Jakarta release the workaround would be to use the out-of-the-box Help Text functionality.
A known problem, PRB1195721, has been opened for this and development has confirmed it's the desired behavior.
"This is the desired behavior. The same happened in platform form as well. Catalog Forms had a bug which was fixed in Jakarta and now both platform form and catalog form have the same behavior where field messages goes away when the filed value is updated."