When using the Advanced Translation Editor any HTML is usually stripped out and in some cases ATE applies markers for those elements, so the ideal Translation Editor for such case would be the Classic Translation Editor, which supports HTML.
Only, in order to do this, you would need to disable the ATE for the whole site.
Manual replacement of the code on translated pages could fix this temporarily but manual edits will get overwritten when translating the content again with the WPML Translation from original content to second language.
What you could try to do, is try to find the iframe markup or at least some of its attributes by using ATE's Search Source field, as explained here:
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If this will not solve the issue, please let me know and I will try to recreate the issue and escalate it internally.
