Native VLAN Traffic Flow on Trunk Ports
When configuring Native VLANs on a switch trunk interface, traffic flow over that trunk port will not be possible by default. Consider the following topology:
Hosts H1 and H2 on VLAN 10 will be able to communicate with each other since VLAN 10 is allowed on the trunk. However, hosts H3 and H4 will not be able to communicate with each other, even though they are configured on the native VLAN 99 and the native VLAN is configured on the trunk.
In order for those hosts to communicate, you must allow VLAN 99 on the trunk, and if you do so, their communication will take place using untagged frames.
Links
https://networklessons.com/switching/802-1q-native-vlan-cisco-ios-switch