I am on Steam Experimental branch currently, in-game version 95094 4.04 and have noticed how it appears to be possible to often add even more slots after having reached the initial limit, by just saving and reloading.
Below an image I created, showing ship data for the various ship types as well as classes.
The bold values for max slot upgrade amounts, appears to be unintended.
Only partly fixed, sadly enough. Behaviour has changed and Cargo does stick to the intended limit. However Tech can still be expanded beyond limit, one at a time. This because for some reason, after switching with Q/E, the button to purchase is active. In both inventory types the next slot does blink, but Cargo has the button disabled with the correct message.
To make expansion easier and not be thrown out the UI completely after each slot, ensure to upgrade Tech to max fist, then save/reload.
Intended values do appear to be source values and are correct for Cargo by the looks of it.
It appears to be fixed now. They also re-balanced max slots a bit more