amp-iframe: intermittent loading (Acast podcast player)

### Description

I use `amp-iframe` to deliver Acast's embedded podcast player, as per [their instructions](https://learn.acast.com/en/articles/4631555-embed-player-amp).

This has been working since I redeveloped my site in AMP around a year ago, but has recently become intermittent - failing and sticking with the spinning loading indicator more often than it resolves.

I've clarified the same behaviour amongst a number of different users, devices, browsers and operating systems, all experiencing the same.

Example code:

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The source format adheres to that given in Acast's example, as do the sandbox attributes and other key values. This was, as I say, consistently working until recently (roughly the last month).

Running #development-1 on the page returns a number of warnings and a couple of errors, but nothing associated to `amp-iframe` that I can see.

Any help in understanding or rectifying this problem would be extremely appreciated.

### Reproduction Steps

Visit: https://www.comedy.co.uk/podcasts/comedians_comedian_podcast/episode-414/

### Relevant Logs

_No response_

### Browser(s) Affected

_No response_

### OS(s) Affected

_No response_

### Device(s) Affected

_No response_

### AMP Version Affected

_No response_

Start by reproducing the intermittent loading behavior at the linked episode-414 page with the provided amp-iframe markup, across the affected environments if available. Inspect the page's development warnings and errors while the player remains on its loading indicator. Done means identifying a reproducible AMP or Acast cause and documenting or resolving the failure.

Written by the indexing model from the issue text.