Hi there!
I’ve encountered some problem with normals and triangulator.
When I triangulate, some normals are become broken. You can see that some of them become different color when they shouldn’t.
Left model triangulated and right is original.
I’m using Blender 2.90.1 on Win 10 Pro.
Auto smooth on\off - same result
Edge Split modificator on\off - same result
Try the Weighted Normals modifier after triangulate (tick Keep Sharp on).
Thanks for the help, but it didn’t help.
I tried to make quads but it looks same when i triangulate. o_O
I’m currently playing with Normal Edit modificator, but it not helps. 
The problem was in a slight ngons bending.
After i scaled to 0 on affected axis, traingulation started to work ok.
