I have a site that I have recently been upgrading from 8.2 to 9.1 and everything is looking good EXCEPT anything that relies on search funcionality.
Any Azure search has in the info log:
2019-02-12T00:16:14 PID Information 6936 00:16:14 INFO AzureSearch Query [sitecore_master_index]: &search=This_Is_Equal_ConstNode_Return_Nothing&$filter=(not search.ismatchscoring('This_Is_Equal_ConstNode_Return_Nothing', null, 'full', null) or search.ismatchscoring('This_Is_Equal_ConstNode_Return_Nothing', null, 'full', null)) and (path_1/any(t:t eq '38eb27a595144c92b00f3f5e277c5de2')) and latestversion_1
I have examined the query just before the iQueryable call to search and can see that the query object seems correct, any facets, or paging etc are all correct.
Note: Search functionality in Sitecore is still working, just not in my web app. I also had to modify my ContentSearch Index config to set IncludeAllFields to FALSE because I went over the 1000 from Azure Search limit.