outlook.office.com - returns IPv6 only

Been using NextDNS without too much trouble for a few months. Configured as DNS on my ASUS router with DNS-Over-TLS configured in strict mode. No issues.

Now I have run into a weird one: 

Trying to get to my company's M365 portal works (portal.office.com), but trying to get to the outlook page (outlook.office.com) fails. The only difference I can see, is that the first returns a set of IPv4 addresses as well as IPv6, while the latter only returns a list of IPv6 addresses.

It's been working until last week or so. 

No local DNS config on clients other than default (= obtain from router).

My browsers only return a "cannot be reached" message when trying to get to https://outlook.office.com

When querying from my Windows 11 and Linux machine, I get no IPv4 addresses returned.  Specifying the DNS servers to query (NextDNS, Google and so forth) it resolves correctly.

Example:

Not working;

host outlook.office.com
outlook.office.com has IPv6 address 2603:1026:c0d:81a::2
outlook.office.com has IPv6 address 2603:1026:c0d:82e::2
outlook.office.com has IPv6 address 2603:1026:c0d:82a::2
outlook.office.com has IPv6 address 2603:1026:c0d:82d::2

Working;

host outlook.office.com 45.90.28.228 ( << Same DNS as configured in my ASUS router)
Using domain server:
Name: 45.90.28.228
Address: 45.90.28.228#53
Aliases: 

outlook.office.com has address 40.99.150.98
outlook.office.com has address 52.98.171.226
outlook.office.com has address 52.98.152.194
outlook.office.com has address 40.99.150.18
outlook.office.com has IPv6 address 2603:1026:c0d:807::2
outlook.office.com has IPv6 address 2603:1026:c0d:803::2
outlook.office.com has IPv6 address 2603:1026:c0d:c02::2
outlook.office.com has IPv6 address 2603:1026:c0d:82e::2
 

Tunneling into the router and querying directly from there works as well. 

Anyone seen something similar? It works when adding a VPN with a different DNS config, and it works when using mobile data and bypassing my own setup. Could be some ASUS weirdness as well of course, and I am digging into that as I write this. Just wanted to ask before I started to change this back to default settings and so on. 

Thanks in advance.