Thanks for your response.
Yeah, the redirection I currently have set up in my old forum .htaccess root directory directs to the right flarum post, but only the ID number shows in the URL (ex. d/postID) and not the whole flarum url with slug. It does load the slug in the URL only if I scroll down the posts page. It adds the slug + a number.( ex. -slug/4).
My old forum's post URL's were /forum/name-of-forum/postID.php. I didn't have a slug for the posts.
The Post Id's for both old and new flarum forums are exactly the same.
My redirection .htaccess file in my old forum root directory looks like this:
RewriteRule [/]+/([\d]+)-([\d]+).php https://www.domain.com/newforum/d/$1 [L,NC,R=301,NE]
RewriteRule [/]+/([\d]+).php https://www.domain.com/newforum/d/$1 [L,NC,R=301,NE]
Like I said above, it redirects to the right posts with the ID in the URL, but not the whole URL with the flarum slug.
I guess if it recirects to the right place, I shouldn;t worry about it, but I have OCD and want it to go to the whole URL with slug.