Hi, I had this problem when using Integrate ove regions:

In[1]:=Integrate[Sin[1*x1 + x2 + x3], {x1, x2, x3}\[Element]Cuboid[{0, 0, 0}, {1, 1, 1}]] // N
(*0.879355*)

In[2]:=Integrate[Sin[1.*x1 + x2 + x3], {x1, x2, x3}\[Element]Cuboid[{0, 0, 0}, {1, 1, 1}]]
(*Sin[1. MeasureDump`X$584557[1.] + MeasureDump`X$584557[2.] + MeasureDump`X$584557[3.]]*)

In[3]:=Integrate[Sin[1.*x1 + x2 + x3], {x1, x2, x3}\[Element]Cuboid[{0, 0, 0}, {1., 1, 1}]]
(*0.879355*)

Note the output of In[2] when I use 1. rather than 1, but restored when I also put in decimal numbers in the region of integration.

When using the usual format for Integrate I got the right answer

In[4]:=Integrate[Sin[1*x1 + x2 + x3], {x1,0,1},{x2,0,1},{x3,0,1}] // N
(*0.879355*)

In[5]:=Integrate[Sin[1.*x1 + x2 + x3], {x1,0,1},{x2,0,1},{x3,0,1}]
(*0.879355*)

In[6]:=Integrate[Sin[1.*x1 + x2 + x3], {x1,0,1.},{x2,0,1},{x3,0,1}]
(*0.879355*)

I have also posted this on StackExchange