All added Parts inSeatInventory where lost by spawn at the Launchpad.

Where these seats occupied by kerbals?

Ok, seats occupied by Kerbals will keep the assigned inventory until the next time the game is loaded.
After that, the seats are empty, though occupied with Kerbals.
If a kerbal wears one of the expansion vintage suits, the same applies but the equip function does not work.

Could you please make and share a short video? Also, double check that you use the latest KIS version (1.11), the game is KSP 1.4, and the MM mod is of compatible version.

The vintage suites issues was addressed in #248. Other than that, I cannot see any problem. Please, re-open this use if you have more info (logs, screenshots, etc.).