The mechanic said it was nothing to worry about, possibly a little bit of water in the system.
I haven't noticed any bubbling for a while.
Apparently it's relatively common with outlanders.
I have a 2016 GX4HS (83k miles).
The coolant is bubbling.
The dealer I bought it from says it's supposed to but I haven't heard of anything like this from anywhere else.
Car does still drive well so maybe the dealer wasn't lying.