Adding IPs to WAN interface cut it off

[whit]

I just added two IPs to a WAN interface, beyond its primary one. When I pressed the button to apply them, the interface got cut off. It can't be pinged on any of the three addresses. And I lost the remote web access to it.

Is this a known thing?

Fortunately I have a WireGuard tunnel to the device which has stayed connected, so I should be able to get to it through that -- although it's from another remote location, so first I need to set up a relaying system there. But what am I looking for here, in terms of what's gone wrong and why?

[whit]

Finally got in, deleted the added IPs, and the original IP is back in business. But what's the problem here? I added them as /24s, since these are on a /24 block which the gateway is .1 of. I've routinely added IPs to Linux boxes for years, and in that context either a /24 or a /32 would have worked, without screwing up anything for the original IP on an interface. I take it there's something more to know about how either OPNsense or BSD adjusts routing when a new IP goes on the WAN interface. But I've no clue what that is, or how to avoid it breaking things as it partially did here.

All hints are welcomed.

[whit]

Any hints at all on how to add virtual IPs without having routing through the static IP already on the interface interrupted? Or is the presumption here that an interface will only have either a static IP or else a set of one or more virtuals?

With Linux, I'm used to mixing them, often using UCARP for the virtuals. Obviously this is a different world. What docs should I be reading to understand this aspect of it?