(8.5.24058.13001, 2024-02-27)
A revolved surface loses its shading after using Trim:
2024-03-04_Trim.3dm (1004.4 KB)
Same with Boolean Union, but not on every attempt.
In pre-V8 it works as expected.
In pre-V7 it works as expected.
Looks like it doesn’t like that one tip of the splitting part falls on the surface seam. If you move the seam away from the joint, it works…
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Yes I know.
To have to use a workaround in this basic beginner model is a shame.
In any case there is something wrong (with the trimming?).
And why does BU work after undo and doing the very same again?
The BU works consistently with the correct shading if you mouse-pick choose first the single leg of the T and then the other crossing part. It fails to shade consistently if you mouse pick the other way around. If you window select, it will depend on whether you window from left to right or right to left… 
Must-know basics 
Absolutely!! 