Thank you for reaching out. I understand you are facing connectivity issues where intermittently you are unable to connect to certain IP addresses from your on-prem services. When you reset the VPN Gateway the IP addresses are reachable again.Depending on my understanding above.
Such issue can happen when there is a routing conflict either on-prem or Azure.
In order to isolate the issue here, when the issue occurs again you can perform a packet capture at your VPN Gateway to check if the packets for the IP address space are going through the VPN Gateway.
When you reset the Azure VPN gateway it reboots the gateway, and then reapplies the cross-premises configurations to it. In order to check if there is any configuration problem or any routing issue you can also use Diagnostic logging of VPN Gateway to validate if any certain event is triggering this issue. Thanks!