openvpn session stalls as soon as OpenVpn loses focus

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I think my (very old now) Samsung S5 is "optimising" OpenVPN and as a result I'm having issues maintaining a usable connection.
I can connect fine and can ping the client from my PiVPN server with ping 10.8.0.2 but as soon as I select another app on my phone and OpenVPN loses focus, the ping timeouts. When I switch back to OpenVPN, the ping resumes. This is only an issue when I'm connected via Mobile Data, when connected via WiFi, ping works fine even when OpenVpn loses focus.

Anyone know what's causing this, or even better what to change on my phone to fix it?

tia

Cheers
Graeme

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I've fixed it.

Settings -> Data Usage -> Select OpenVPN in the list of Apps, untick "Restrict background data".
I have no idea how it got ticked but suspect my phone got over enthusiastic when I let it automatically optimise my apps a while ago.