Don't forget she has some weird ass notions about most things. On top of her not-so-fun background, she seems to have a tendency to take things a bit too literal (e.g. the maid handbook), so if the MC called her his pet before, it might just be she took this too a bit too literal. Frankly even though it looked weird, what I gathered from the scenes with her it actually didn't surprise me that much. Afteral, she does have some weird notions about relations (e.g. master <-> maid) and the "real world". Plus her time at the "massage parlor" may have caused her to be mentally 'different' as it was hinted at before that she didn't get treated normally during her stay there
EDIT:
MoonBox You probably already got messaged about this, but if you've ended Tanya's path (choosing "Pulled out" when MC thinks about his last encounter with her) the
tiaPreg
var is never set, causing an ignorable error when starting Part2 and continuing Part1's save or loading the last save from Part 1 directly