The Art scroll siddur uses the Shehakhol nihyeh (present) . The shulchan aruch says Nihya (past tense). Which is to be used by Ashkenazi Jew and why is there a deviation from the Shulchan

Shalom, Thank you for your question. There is much discussion amongst the rabbis as to the correct version of the blessing "shehakol" – as you write, some say "nihyeh" and others "nihyah". The law is that both are acceptable and one should follow their family or community custom. For example, on the "nihyah" side are the Shulchan Aruch HaRav of the Ba'al HaTanya (204,18), the Chatam Sofer, and the Kitzur Shulchan Aruch (52,2). And on the "nihyeh" side are the Gr"a, the Chai Adam (49,1), and the Aruch HaShulchan (204,1). (Source – Piskay Tshuvat, 204,1). I am not sure what you are referring to when you say that the Shulchan Aruch uses "nihya" – as far as I know the Shulchan Aruch itself does not refer to this question at all, and does not express an opinion about it. Perhaps you are referring to the (above mentioned) Shulchan Aruch HaRav of the Ba'al HaTanya (204,18). If so, then it is not surprising that different siddurs might use a different version. This is because, as opposed to the Shulchan Aruch itself, authored by Rabbi Yosef Karo (about 500 years ago), which has much authority, the Shulchan Aruch HaRav of the Ba'al HaTanya (about 200 years ago), whilst being a very important halachic work, is only one of many of the works of the later Rabbis, and does not impart the same level of authority. (Though Chabad Chassidim generally follow it, other codifiers, such as the Mishna Brurah, view it as one important work amongst many). I hope this is of some help to you. Blessings.