I've just tested for you on my Alma8, CWP 1.7 & a new subdomain was created without an issue using CWP UI. I used the below details and it created the subdomain + the folder for me.
Add SubDomain: newsub
Folderpath: /public_html/newsub
I didnt tick the AutoSSL box though as for that to work I'd need to add a DNS record for the 'newsub' subdomain. I deleted it just as easy.