Open question to the floor.
Lots of English words use ‘in’ as a qualifier. Inadequate, inconsequential,indigestion, etc.
What all those words have in common is that the qualified word stands on its own and is made negative by the addition of ‘in-”.
But what about ‘tense’. In-tense does have a qualifier, but here ‘in-’ makes ‘tense’ stronger.
English. An annoying and confusing language.