Bug introduced in 12.0 and Fixed and fixed in 12.1
After upgrading to mathematica 12, this integral gives the wrong result, also with other types of similar integrals.
Integrate[t[2]*Exp[t[2]],t[2]] The output
Exp[t[2]^2]/2 is wrong (it should be E^t[2] (-1 + t[2])). Such problem does not exist in previous versions.
PS, I have to use a lot of t[i] in my program, even if I change the [i] to subscripts, the problem is still there.