tfjs: Inconsistent ”Could not get context for WebGL version 2” error message

System information

• Have I written custom code (as opposed to using a stock example script provided in TensorFlow.js): no
• OS Platform and Distribution: macOS 10.15.7 and iOS 14.1
• Mobile device: iPhone 11
• TensorFlow.js installed from (npm or script link): https://cdn.jsdelivr.net/npm/@tensorflow/tfjs@2.7.0
• TensorFlow.js version: 2.7.0
• Browser version:
  • Safari 14.0 on iOS: Mozilla/5.0 (iPhone; CPU iPhone OS 14_1 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/14.0 Mobile/15E148 Safari/604.1
  • Safari 14.0.1 on macOS: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/14.0.1 Safari/605.1.15
• Tensorflow.js Converter Version: ?

Describe the current behavior

Steps to reproduce:

1. Load https://js.tensorflow.org/api/latest/#add

2. Click the first Run button

3. Console log prints:

Could not get context for WebGL version 2
Tensor
    [11, 22, 33, 44]

1. Click Run again
2. Console log prints:

Tensor
    [11, 22, 33, 44]

Describe the expected behavior

Expected the log messages in steps 3 and 5 to be identical, including the WebGL error message.

Standalone code to reproduce the issue

Reproducible test case: https://js.tensorflow.org/api/latest/#add

Other info / logs

The latest stable Safari 14 ships with WebGL 2.0 disabled by default. Make sure WebGL 2.0 is disabled via Safari > Experimental WebKit Features to be able to reproduce this issue.