Post Update destroys edited text

This has made WordPress nearly unusable at my site.

I write an Post, the Text and Visual tabs both show correct edits, and say word count is 631.

I click on Update.

It shows the Post truncated! Update throws away a large part of the article – it reduces it to 116 words, or 233 words.

All I can make out at this point is that it allows only a certain number of these html tags:
<span class="entypo">& #128640</span>
in the text. (Spaces added to character above to make it show correctly here.) The character does not seem to matter. It keeps the first four such instances, and starting from the fifth instance, WordPress deletes it and all text following it.

Moving the fifth instance moves the truncation point, and changing the character used has no change. Removing all 5th and higher span tags and keeping only 4 of them keeps my entire article and does not truncate it. But these numbers are not fixed – I added three more span tags before the last paragraph, and at least for now, it has not truncated the last paragraph.

Stumped! Any ideas on how to work around this bug or how to fix it?

I’m using the Customizr theme and have only a handful plugins.