As I understand it, you should have <description> mapped to a paragraph style, and <em> and <strong> mapped to character styles. Is that what you have set up?
When you say "each time there is a change," that suggests the import works initially but then when you re-import the same XML you lose data? Is that correct?
It should not matter what your tag is named.
Lets see if I can explain this better:
Yes, we can map to character styles. And we can import an xml document with these elements, and manually tag them <em> or <strong> in InDesign, and it works. If we re-import the same exact xml, or slightly different xml but with the tagged elements in the same order and number, it works.
If we import new data with <em> or <strong> tags that haven't been pre-tagged in InDesign, we lose the data. We have to go back to the InDesign document and manually retag the element. If we switch the order of <em> or <strong> in the data, the text is dropped, and we have to manually tag the new elements into the InDesign document.
ex #1: <description>Here is our first sentence in <strong>bold</strong> then <em>italic</em>.</description>
if I tried to change the data and reimport a slight change:
ex #2: <description>Here is our first sentence in <em>italic</em> then <strong>bold</strong>.</description>
If we changed the data in the xml as in example 2, and try to reimport this, the data between the <em> and <strong> tags completely dissappears.
InDesign treats the <em> and <strong> tags as xml elements, which makes sense after all. We need a finer level of granularity.
I hope that makes sense!