Swagger UI not rendering errors

Q&A (please complete the following information)

• OS: Windows
• Browser: Chrome
• Version: 67.0
• Method of installation: dist assets
• Swagger-UI version: 3.17.x (potentially lower versions are affected as well)
• Swagger/OpenAPI version: OpenAPI 3.0

Content & configuration

API being split over several files
api.yaml

openapi: 3.0.0

info:
  version: v1.0
  title: Testing
  description: Testing

paths:
  /testA:
    $ref: 'testA.yaml'
  /testB:
    $ref: 'testB.yaml'

testA.yaml

get:
  responses:
    200:
      description: OK
      content:
        application/json:
          schema:
            $ref: 'non_existant.yaml#/schemas/testA'

testB.yaml

get:
  responses:
    200:
      description: OK
      content:
        application/json:
          schema:
            type: object
            properties:
              id:
                type: string

Describe the bug you're encountering

The red errors message field is not rendering. The browser console (when using inspect) however still shows the correct error about not being able to GET the the referenced resource (404 Not found).

Interestingly enough, when commenting out the path reference for testB, the error caused by testA is rendered as expected.

Judging from the intial rendering, it seems like the error is temporarily shown, then vanishing. However im not completely sure about that.

Expected behavior

Errors should be rendered.

Reproduce the issue using the provided api.yaml, testA.yaml, and testB.yaml with Swagger UI 3.17.x and the dist assets. Compare rendering with both path references present and with testB commented out, while checking the browser console. Done means the red error for the missing referenced resource remains visible when both paths are loaded.