Its the even number of voting members that are the issue.
It looks like the main deployment is UK. I’d expand this to 3 voting members.
This really does read as a networking type issue. Bandwidth, traffic shaping, one route prioritized over another. I can’t think of anything specific to MongoDB that would influence this.
I posted this parameter in response to the very first post with 2 secondaries in the remote region where they are resetting to sync from the Primary when the secondary sync source is lagging more than 30s, perhaps it could help that situation.