Since she only wees in the dining room when no one is in there, it does sound more like a learned habit or a “this spot smells like a toilet” association rather than a housetraining problem or a medical issue (especially since you’ve ruled health out). Even with enzymatic cleaners, some dogs can still pick up the tiniest residual scent, so to her that room may still be flagged as “the place to go.”
A few things could try:
Reset the association with supervision + reward, For a couple of weeks, whenever the dining room is open, keep a very close eye on her. The moment she enters and doesn’t wee, reward her for calm behaviour in there , treats, praise, scatter feeding, a toy. You’re basically teaching her: “This room is for hanging out, not going to the loo”
Block access unless you can supervise
Add a temporary ‘job’ in that room
Feeding her meals in the dining room or giving her a stuffed Kong in there can help shift the mental association away from toileting.
Increase garden toilet opportunities before she’s allowed in
Think about boredom or understimulation
Collies are very bright, high-drive working dogs. Even when they seem calm and chilled, they often need more mental work than the average breed. Sometimes boredom or a lack of “jobs” can lead to quirky behaviours
Hopefully a short reset plus some mental stimulation will break the cycle!