Hi,
1) before you wrote:
"Fixed the "read more" issue by adding the code in wplms customizer plugin :
add_filter('wplms_course_excerpt_limit','my_custom_wplms_course_excerpt_limit');
function my_custom_wplms_course_excerpt_limit($limit){$limit = 99999;return $limit;}
And the content html issue was resolved by correcting the image link in the text mode in back end editor."
If there is a theme bug related to Read More tag, should I remove this code from wplms plugin?
2) Also, how do I save the customisation made in wplms customiser plugin upon the theme update (it will be overwritten with all default settings)?
No the code is to disable the "read more" functionality in wplms.
And no the code will not be removed after the update of wplms as wplms-customizer plugin is provided for the customizations and never gets updates .
.