LibreOffice Writer Mendeley {Formatting Citation} frozen, any idea?

I am using LibreOffice 6.1.4.2 with Mendeley 1.19.3 on Ubuntu 18.10.

This is quite bothering me that when I cite a reference from Mendely, the citation just freezes and the {Formatting Citation} just can turn to your citation, nor does the inserted biobliography update with new reference. Here are more details for this bug:

1. No problem at all with a new .odt file.
2. Since there are no problems with the new file, I copy/paste everything to a new file so I can continue to add citation based on the previous ones, everything is OK, but after adding around 20 citations, the new one gets frozen again. Which means I need to paste the new file to a "new new" file (if I really need to).

I found some similar problems for other people on other websites but it seems there is no such a satisfactory anwser. So I hope some one can give me a hand.