This thread is resolved. Here is a description of the problem and solution.
Problem: WPML is stripping heading tags in WooCommerce product description added by Yoast Solution: All html tags in this field were removed on saving a translation of a WooCommerce product.
In itself the category description has no WYSIWIG editor, so only plain text can be entered.
Removing filters on this field has now resolved this issue.
The following thread gives some more pointers to this issue: https://wpml.org/forums/topic/how-to-allow-html-in-category-descriptions/
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