I found a bug with org-mtags.el. The table of content tag in muse is <contents>, but in org is <content>.
<contents>
* title1
* title2
----8<----8<---
then export using "C-c C-e h" will lead to a loop in org-mtags-get-tag-and-attributes.
I wish I can use <contents> to be consistent with muse, and get a warning from org if the tag is not supported.