Thankfully there is a button to move components and stuff from the char stash to the global stash. This process will merge them.
If I do this manually - for whatever reasons - e.g. with potions they will be placed in addition.
I would like to suggest that items should items should always merge / stack when putting them from one stash to another or from one global stash tab to another.
Thanks for your time. Indeed holding shift while clicking does stack.
Also tried the behaviour when putting things from one tab to another in the stash.
This request can be marked closed now, if the shift behaviour is the wanted one.
Personally do not see a sense not to stack but there might be usecases I miss to see.
I believe if you are also manually moving them one by one and place them over the top in the stash they should stack. Certainly works if you want to take just one item from a stack and place in another stack elsewhere.