Learn Turkish: Bu zihniyeti garipsiyor olmalı. - He must be finding this mindset strange.

It comes from the adjective garip (strange). By adding the derivation suffix -se (a variation of the -(I)msE suffix you are learning), it becomes garipsemek, meaning 'to find strange'. The final e in garipse narrows to i when we add the continuous suffix -iyor, resulting in garipsiyor.

Why does this word use just -se instead of the full -imse suffix seen in other words?

The perception suffix adapts depending on the root word. While some words take the full -imse (like ben becoming benimsemek), certain adjectives like garip or hafif just take -se or -sa. It serves the exact same function, showing someone's attitude or perception towards the object.

Why is zihniyet marked with the accusative suffix -i?

The verb garipsemek needs a definite direct object because the subject is finding a very specific thing strange. That specific thing is bu zihniyet (this mindset), so the accusative suffix -i is added to make it bu zihniyeti.

How does the -iyor olmalı structure work to mean 'must be finding'?