As Michael said this issue might be caused by indexing as sitecore uses the master index to get these results. If you take a look at the Search.log files you will see entries like these:
Query [sitecore_master_index]: &search=((template_1:(f1828a2c7e5d4bbd98ca320474871548) OR template_1:(daf085e8602e43a68299038ff171349f) OR template_1:(c97ba92380094858bdd5d8be5fccecf7) OR template_1:(eb3fb96cd56b4ac997f8f07b24bb9bf7)) AND language_1:(en))&$filter=(path_1/any(t:t eq '3d6658d8a0bf4e75b3e2d050fabcf4e1'))&queryType=full&facet=calculateddimension&facet=templatename_1&$skip=0&$top=20&$count=true
Can you try to rebuild your sitecore_master_index
and see if the issue is still happening? Depending if you use SOLR or Azure you should also verify the configuration of the master index in \App_Config\Sitecore\ContentSearch\Sitecore.ContentSearch.Solr.Index.Master.config
or \App_Config\Sitecore\ContentSearch.Azure\Sitecore.ContentSearch.Azure.Index.Master.config
to see if the media items are being indexed:
<locations hint="list:AddCrawler">
<crawler type="Sitecore.ContentSearch.SitecoreItemCrawler, Sitecore.ContentSearch">
<Database>master</Database>
<Root>/sitecore</Root>
</crawler>
</locations>