Maybe even if it's just aging / maturing, I'll keep with treating randomly and the voluntary check in, just to reinforce my value, and coming to me.
That sounds like a good plan! Also when calling him, try lowering criteria, maybe try calling him when he has been in the yard for some time and is getting a bit bored and maybe even a bit thirsty. I wouldn't call him in the house without checking what he's doing first. If he's actively sniffing consider that a good part of the brain is used for that, so it may be more difficult grabbing his attention right away.
However, make sure to protect your recall, because calling too often with no response can lower its value considerably and can lead to learned irrelevance. You many need to go back to basics to power it up again, in areas with less distractions (avoid carpet areas though) and re-introduce valuable rewards (tossing the treat to prevent shock).
I like to call dogs for their meals taking advantage of a dog's eagerness to eat, even if its offered in a food puzzle. However, I aim to get a response without showing food to not fall into the bribery trap. So treats are in pockets and meals are hidden behind my back (or I remain hidden in a hide-and-seek fashion), or even later, the food may be left on a tall shelf or counter so when the dog rushes to me, we both run towards this location, I ask for a sit and the food puzzle/bowl is placed on the floor.
If Loki though is suspicious of certain foods because he has associated them with shock, be careful as that'll risk putting a dent in the recall too, so use something you're sure he loves and hasn't been "tainted' by any shock.
While it's tempting to lower expectations once a dog appears to be better trained, relaxing a bit too much leads to setbacks that are then reflected in the level of a dog's responsiveness This is a time when more training can turn extra handy, to keep that teenager brain busy although you may notice times of regression. It is great that you have plans to up the training coming the next weeks.
Something else to factor in, is that between the ages of 6 to 14 months, dogs go through another fear period:
https://www.braintraining4dogs.com/members/archive/fear-periods-8-10-weeks-6-14-months/