Nobody else seems to be having this issue by the looks of it.
Nonetheless, here are the results of my testing:
Mi A2 Lite:
(recently factory reset with no other apps installed or settings changed other than the initial set up)
Connected to Wifi, with no SIM, the standby drain is <0.1%/hour. Adding in the Telkom sim, the drain is still <0.1%/hour. Disconnecting from Wifi and using Telkom LTE increases the standby drain to 0.2-0.3%/hour. Swapping out the Telkom sim for the MTN one and using the Afrihost APN sees the standby drain surge to around 2.4%/hour regardless of whether I am connected to a Wifi network.
Each case was tested over a 24 hour period and done more than once.
Oddly enough, switching off the mobile data, but still using the Afrihost APN, reduces standby drain to 1.2%/hour. I was expecting it to go down to about 0.2%/hour, because the APN should not be in use.
Galaxy S7:
On the Afrihost APN, the standby drain was 5-6%/hour, according Accubattery. After switching to APNs to MTN, it now hovers around 3%/hour.