@comchia Hmm... I'm on Windows 10 and just installed Pico-8 0.2.4b. I put in 3 decimal values (1.123, 2.234, 3.345) into cells A1, A2, A3 and in A4 entered @sum(a1..a3) (note: two dots between cell labels, not three)
Next, I put in horizontal entries using very large decimal values: 1.123456789, 2.23456789, 3.3456789486 and ran a summation on those to properly receive 6.7037(etc...)
So far, it is working as expected for me. Could you please provide more detail about the values you entered and the summation formula you attempted?