Delegated subdomain DNS not working for one particular site

(Sorry, had to replace lots of dots with ‘-DOT-’ as this board doesn’t allow posting links)

I have a mydomain-DOT-com that uses Cloudflare for DNS but wanted to delegate the DNS of a subdomain mysubdomain-DOT-mydomain-DOT-com to a different one (shared hosting server on Namecheap, which already has that domain/subdomain set up in cPanel), so I added the NS records dns1-DOT-namecheaphosting-DOT-com and dns2-DOT-namecheaphosting-DOT-com for the subdomain in Cloudflare DNS.

However even after a day has passed the subdomain mysubdomain-DOT-mydomain-DOT-com doesn’t resolve (on a related note, the same setup for anothersubdomain-DOT-anotherdomain-DOT-com started working immediately yesterday, so for some reason it only doesn’t work for subdomains in mydomain-DOT-com).

Below are the last few lines of “dig +trace mysubdomain-DOT-mydomain-DOT-com @”:

mysubdomain-DOT-mydomain-DOT-com. 300 IN NS dns1-DOT-namecheaphosting-DOT-com.
mysubdomain-DOT-mydomain-DOT-com. 300 IN NS dns2-DOT-namecheaphosting-DOT-com.
;; Received 124 bytes from in 421 ms

mydomain-DOT-com. 3601 IN SOA dns1-DOT-registrar-servers-DOT-com. hostmaster-DOT-registrar-servers-DOT-com. 1580327645 43200 3600 604800 3601
;; Received 139 bytes from in 423 ms

And below are the last few lines of “dig +trace anothersubdomain-DOT-anotherdomain-DOT-com @” (the one that works):

anothersubdomain-DOT-anotherdomain-DOT-com.	300	IN	NS	dns1-DOT-namecheaphosting-DOT-com.
anothersubdomain-DOT-anotherdomain-DOT-com.	300	IN	NS	dns2-DOT-namecheaphosting-DOT-com.
;; Received 109 bytes from in 420 ms

anothersubdomain-DOT-anotherdomain-DOT-com.	1200	IN	A	SHARED.HOSTING.SERVER.IP
anotherdomain-DOT-com.		1800000	IN	NS	dns1-DOT-namecheaphosting-DOT-com.
anotherdomain-DOT-com.		1800000	IN	NS	dns2-DOT-namecheaphosting-DOT-com.
;; Received 125 bytes from in 424 ms

What’s wrong with mydomain-DOT-com?