BGP Case Remove /24, Hoping took over by /23

Purpose : Load Balancing Traffic
Schenario : Moving /24 from Juniper (connected to Upstream 1) to Cisco (conncted to Upstream 2) Core Routers.
Steps :

- Remove /24 from Juniper
- Keep /23 on Cisco
- Clearing BGP session on both

Looking-glass & Route-Server outputs still hold /24 on their routing table.
Crosscheck to Direct Upstream no more /24.
Issue ? Why /24 holds by each Route-Server/Looking Glass server outside ?

An idea to come : Create manually /24 on Cisco Core and readvertise to Uptream 2.
Done, remove /24 and clearing BGP session remains similar routing table outside.
Pretty strange againts normal changing.

Some said, it takes even 15 minutes to take effect if above steps not a choice for you ?

rgs
a. rahman isnaini r.sutan
tks, to IWI for share.