Host_cache null or wrong Host

[Dan Kelly]

We're having problems with the host_cache table on our MySQL 8.0.35 / Windows Server 19 instance

We've recently updated our users so that the "From Host" is "%.domain.com" rather than just "%"
However we are seeing two, possibly related issues, where users either being rejected or are being allowed in with the wrong IP address.

Scenario 1:
User is rejected with "Host xx.xx.xx.xx is not allowed to connect" errors.
Checking the performance_schema.host_cache the IP Address is listed but has a NULL entry in the Host column

TRUNCATE TABLE performance_schema.host_cache generally resolves the issue

Scenario 2:
User is connected over the VPN and gets the same error as above.
Check host_cache and NULL entry.
Disconnect from VPN and reconnect at some point later. Connection is now granted

*However* the VPN has assigned a new IP address to the connection and checking host_cache that IP is assigned to a different Host than the current host. In our currecnt scenario this isn't an issue but could become one.

Both scenarios suggest that the host_cache is not refreshing the correctly. What are our options?

We'd prefer to use the Domain Name as we have a fairly complex array of subnets that users could connect from depending on Location/Wifi/LAN etc.

At this point we are flushing the cache 2-3 times a day which feels wrong. Generally the cache is well below the limit of number of records.
How much of a performance hit would disabling the host cache be?