Not All Corporate DNS Zones Get Resolved Via VPN

We run a split DNS environment, same (plus additional) DNS zones in LAN as in WAN, with differnt private vs. public IPv4 addresses LAN vs. WAN. If like there is a DNS host.company.com in LAN and WAN DNS, it has 192.168.0.10 IP in private DNS and 213.66.12.10 in public DNS. Addditionally there is like a zone company.intra which obviously only exists in LAN:
How can it be explained that it often happens that while being connected to our company LAN via VPN (SSL VPN) we can resolve correctly with LAN IP all .company.com hosts, but company.intra hosts cannot be resolved at all. While company.com host is the same (same IP) as company.intra host. And the host can then be pinged either as host.company.com or LAN IP, but not as host.company.intra.

but a nslookup -d2 host.company.intra works, the right DNS server (not my ISP one, but the one from the company) is queried and the result is correct. But why can I then still not ping this host (unknown host), or reach it like http or so? It is not a firewall or port issue, assume that.

As far as I can tell, it looks like a client PC reboot always helps, so that also company.intra DNS starts working.  Any idea where I can start looking at? It must be a pure client-side issue, but it occurs quite often and is annoying

kind regards,
Dieter