It happens very often in Polish that a word ending with "ó" + consonant loses the accent on the "ó" whenever an ending is added. For instance table/tables is translated as stół/stoły and there are many other examples.
Just get used to it :-)
There are other common changes;
'ą' goes to 'ę': ząb, zęby (tooth/teeth)
'io' or 'ia' goes to 'ie': sąsiad, sąsiedzi (neighbour(s))
And as you might have noticed the 'e' before the final consonant often gets dropped like pies/psy.
The same thing -- i.e., a change in pronunciation in a different form of the word -- happens in English too, although it's not necessarily reflected in the spelling (because English is such a messed up language, orthographically speaking). A few examples: child --> children; woman --> women; louse --> lice; thy --> thine; vine --> vineyard.
It is not an accent mark, it is a diacritical mark. "Ó" is a different letter with a different sound.
Diacritics in Polish represent different sounds, never accents. mój becomes moja exactly for the same reason, why, for example, lesen becomes liest (German) or volver becomes vuelvo (Spanish).
This is just an irregularity. It emerged some time ago and stayed in the language. It could be for various reasons, for example easier/more natural pronunaciation. Maybe this helps:
So the herbata is feminine then? If so, "mój herbata" is not gramatically correct?