This bug report has been made into a TODO, since it's a known limitation which would be good to address.
Currently the resulting surfaces from beveled curves contain duplicate vertices and edges along seams.
We could share vertices between these surfaces to avoid problems like the one from this report, where modifiers don't handle duplicate geometry very well.
See the example below.
Operating system: Darwin-18.7.0-x86_64-i386-64bit 64 Bits
Graphics card: AMD Radeon Pro 455 OpenGL Engine ATI Technologies Inc. 4.1 ATI-2.11.20
Broken: version: 2.81 (sub 16), branch: master, commit date: 2019-10-28 18:34, hash: rB7c1fbe24ca33
Short description of error
When having a BezierCurve with a "Extrude" and a "Depth" value and you add a Solidify Modifier to it, you will get an error in the geometry between the straight surface and the radius. It is either intersecting geometry or a gap.
Exact steps for others to reproduce the error
Add BezierCurve . Give some value to "Extrude" and "Depth" in the object properties panel. Add a Solidify Modifer.
Please see attached .blend file.