A valve guide gets loose because of faulty build quality from the factory.
Oil changes and gas brand do not cause valve guides to free themselves from their interference fit in the cylinder head casting.
You might be able to take this up with Subaru, and get them to partially pay for the repair of the engine.
BC.
Dealership will make some adjustment to bill, yet to be determined. Difficulty with going after Subaru is that car is well out of warranty–purchased in 2004 ( 2005 model) so nearly 7 years old with 79,000 miles. I also had head problems with a former Subaru, blown head gasket, and extra expense with tires ( they don’t recommend any tires to be different in wear, so you often end up buying a full set). Not sure I would buy again
I’m in agreement that a dropped valve guide would likely be a manufacturing defect. The only other reason for a dropped guide would be if the engine was severely overheated. When this happens sometimes the head material will crack between the valve seats so this could possibly cause it.
You stated that your other Subaru overheated but nothing about this one so I assume it was not overheated. This would fall back onto a rare manufacturing glitch as a dropped guide is practically unheard of on a Subaru.