Having the following CSS applied would solve the issue:

.tux-proofread-translation, .tux-proofread-source { white-space: pre-wrap; }

I would like to prepare a patch for this.

Prepare the patch
www.mediawiki.org/wiki/Gerrit/Tutorial

Change 108330 had a related patch set uploaded by Tinaj1234:
Added CSS to preserve whitespace

https://gerrit.wikimedia.org/r/108330

Change 108330 merged by jenkins-bot:
Added CSS to existing selector: preserve whitespace in review mode

https://gerrit.wikimedia.org/r/108330

Patch merged in comment 5 - assuming this bug is FIXED.
If that is not the case: Please reopen and elaborate what is left to do here to get this report fixed.

Verified with the provided link (test-case)

Any reason for not also doing this in page mode?

.tux-pagemode-source, .tux-pagemode-translation {
	white-space: pre-wrap;
}