Dog Howling briefly when left alone

Hi all, we have a 2 year old Doberman/dalmatian mix and a 5 year old border collie. Almost every time they are left in the house throughout the day they howl for about 1-1.5 minutes and then stop and lay down the rest of the time. It’s only 1 howling session, usually 10-15 minutes after we leave and then they don’t do it again. The dogs are not destructive or using the bathroom inside when we are gone. They are “crate” trained to a bedroom that they stay in when we are gone. They have never tried scratching the door or make any attempt to escape. They don’t pace or seem freaked out. They also never howl at any other time, they bark if the doorbell rings and sometimes my border collie barks at people walking right by the window. (There’s a camera in the room so that’s how we know their behaviors)

We aren’t sure if it’s really separation anxiety, but we are leaning that way because the 2 year old has become very uninterested in treats/puzzles/toys when we’re gone and will only eat them when we get back. He is the one that starts the howling every time and then my border collie joins in. We’re working on exiting and returning in smaller increments, they don’t have trouble with stay/place and will hold it even when I’m out the room. But it feels like Dash (the 2 year old) just has to have his minute of howling when we leave. If we talk on the camera at anytime it confuses them and they start barking and get very worked up so we don’t do that anymore.

Honestly if we were in a house I wouldn’t care, but we are in on post housing currently in a townhouse style home so we share 1 wall with a neighbor. There is a bedroom between the room that the dogs stay in and the wall we share, so I’m not sure how much the neighbors actually hear. The howling is loud enough that you can clearly hear if you’re standing outside, but not when I’m sitting in my car outside the house. But it’s something we want to address because we don’t want to start getting complaints. And just for the future if we move into an apartment where we have more than 1 neighbor.

Has anyone dealt with this exact scenario or something similar? Any tips?

TLDR: 2 year old dog and his brother howl for 1 minute when left alone and then stop. They don’t howl again after that and will lay down/go to sleep. Happens 10-15 minutes after being left alone, have tried exercise and frozen treats, which the 2 year old dog no longer eats. No other destructive or anxious behaviors when left alone.