If I read this correctly, you can still find bold / italics in documents created in the last couple of days, but not in older ones.
Could you please make a copy of an older file, cut out most of the text leaving a few examples of the problem. Upload it so we can try to recreate the problem, to confirm it is not just local. Then you can submit a Feedback to MS reporting this bug
Upload Example - Trouble Shooting - Share OneDrive File
Trouble shooting problems in files can be like a visit to the dentist, a long, slow painful process of us trying to “extract” the clues needed to recognize the problem in a back and forth flow of questions and answers so that we can come up with a / “the”
solution.
Often it is faster and easier for everyone if we have a “sample file” get "hands on", to look at, and to “play with”.
This next link provides some tips on setting up a sample file and specific instructions for uploading and "sharing" it for us to access:
https://answers.microsoft.com/en-us/windows/forum/windows_other-winapps/trouble-shooting-share-onedrive-file/a231a097-bcbf-4e34-ad6c-a33118baf471?tm=1523189328156
Includes a link to a macro to randomize text in Word.
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Send Feedback to MS Developers ****Launch Feedback Hub via
<WIN><F>
“Official” MS does not pay much attention to this forum. However, you now can send your complaints and feature requests directly to MS developers and managers using one of the 4 possible “Feedback” mechanisms
described in this link:
https://answers.microsoft.com/en-us/windows/forum/windows_10-windows_install/send-feedback-to-ms-developers/506e7654-8ced-405b-911e-c58d8f253069
Please post a share link to your feedback, back here, so other people who find your question will be able to vote for it and add their comments to it.
Collection of MS “UserVoice” web links:
http://answers.microsoft.com/en-us/windows/wiki/windows\_10-desktop/ms-suggestion-box-sites-at-usdervoicecom/7397119c-6d48-49e6-8d94-7bd0a50d214f
MS Feedback article, says basically the same thing:
https://support.microsoft.com/en-us/help/4021566/windows-10-send-feedback-to-microsoft-with-feedback-hub