There could be scenarios where panel is failing to update IP within the vps.
This would happen in cases where the vps is manually migrated either from other Virtualization server or it was created manually or migrated and imported from SolusVM panel.
In this case, vps IP changes on panel but not within vps.
You will need to look for OSTEMPLATE option in the VPS config file :
#OSTEMPLATE="ubuntu-16.04-x86_64new"
You can un-comment it : (example VPS config file location /vz/private/22026/ve.conf)
OSTEMPLATE="ubuntu-16.04-x86_64"And then you can try to change vps IP again from panel and see if it helps to update within vps or not.