On export to ePub and / or html (on Windows 10 64-bit), single characters are removed from class names. This appears to happen randomly (as shown below), but consistently (the same tags will have the same errors after closing and opening the document, after quitting and restarting InDesign, after restarting the computer, and whether exported as ePub or html).

– span class="Colour" becomes span class="Colo r" (set on character style)
– h4 class="Event-Month" becomes h4 class="E ent-Month" (though h4 class="Event-Date" is OK) (both set on paragraph style)
– span class="Link" becomes span class="Lin" (set on character style)

These same errors are not restricted to one document; they also occur to another document with the same style sheets.

Changing these names revealed that this issue is related to a specific character in the class name, rather than the number of characters, as follows:

– changing to span class="Clr" outputs span class="Clr"
– changing to h4 class="Evnt-Month" outputs h4 class="E nt-Month" (h4 class="Event-Date" remains OK)
– changing to span class="Lnk" outputs span class="Ln"

This means that, for some reason, random characters in specific class names are being removed or replaced with spaces.

Timing: I've updated my Windows 10 Pro to version 20H2, build 19042.630, but I've looked at a backup file created before that update and the issue was still present - therefore it's not related to the recent Windows feature update. The issue was not present last month, when I exported a document with the same settings, using InDesign v15.1.3 x64 - so it's happened after that. Therefore, this issue must be caused by a bug in the 2021 version of InDesign (v16.0 x64). I have confirmed this by exporting the same document as last month in v16, and the same error is present in that exported file.