File field overriding when using Media Manager

I have a File Download field setup and I would like to be able to select the files within the Joomla Media Manager (there are over a thousand and are already uploaded within the file system). 

The field is set to allow multiple values and I am able to select the files. When I add a second value, but when I select the file, it overrides the selected file in the first value. 

I tried this with the file field on it's own and within a group, but have the same results with both.  Is there a setting or something that is causing this?

Thanks

Flexi - Latest Github
Joomla - 5.1.4

Edit:
I currently have the field set to 'Inline uploads + My Files reuse modal (with: URL links + Joomla Media manager)'. When I use 'Joomla Media Manager' I am only able to view images and the files I need are PDFs - this setting may work if I could view the PDFs.

I may have solved this by removing JCE as the default media manager. I am able to view all documents and they are no longer overriding the first value.

ok thanks for retun