How does an internal liquid sensor get tripped if the external one is fine on an iphone 6??

Took my iphone 6 into the Apple store in Market Mall (Calgary) to get the battery changed. The phone works reasonably well for a 3 year old phone, but the battery issues are getting annoying (slow loads, battery life goes down really fast, sits at 1% for ages, etc.). I waited a few weeks to get the call about the battery coming in When I brought it in they checked the liquid sensor in the sim card slot and it was fine. Came back two hours later and they hadn't changed the battery because a second liquid sensor on the inside had been tripped.

First off, when there has been so much bad press about the shady way apple had been handling the battery degradation problems in older phones, you would think that they would be eager to make customers happy by doing such a small repair regardless of the phone state;

Second, how does an internal liquid sensor get tripped if the external one is fine?? I keep my phone in an otterbox case ALL THE TIME. I have no idea how or when the sensor would/could have gotten tripped.

Third, the only solution offered was to do a full handset swap for $390. I could get a new iphone 8 for WAY less than that rather than swapping out a 6 for another 6. ***

My main QUESTION I guess is whether anyone else has ever experienced having one sensor get tripped but not the other and how does that even happen?

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<Edited by Host>