5 month old resource guarding?

I have a 5 month old male olde english sheepdog x poodle mix (sheepadoodle). behavior wise he’s quite wonderful, he’s been easy to train and eager to please, very food motivated. when we feed him we usually are doing other tasks either in the same room or in a different room, point being we don’t interact with him as he eats. when he’s chewed on stuff in the past i would occasionally pet him but quite rarely. today, he was working on a chew and our older senior dog was walking by him (walked past him didn’t interact or look at him) and he growled at her. my partner and i were raised an eyebrow and thought it was unlike him. just to test the waters we gave it a few mins of him calmly chewing, walked past him and he had no reaction and continued chewing happily and relaxed. great. we decided to up it to see where his boundaries were, i walked by him and reached down to scratch his head and he started to growl at me!!! i immediately stopped and left him alone and didn’t react leaving him to finish his chew time.

Is this normal behavior? I understand dogs do resource guarding but I’ve never owned a dog that growls at me in any capacity — i also recognize that resource guarding can get worse and to address it early. What do we do? Thank you!