Clustered Index Scan on 5+ billion rows instead of using non clusterd index for <1000 rows
Sometimes the person writing T-SQL has more accurate information than the query optimizer. In all of your query plans, SQL Server has a final cardinality estimate of 5360890000 rows, which is how many rows are in the dbo.SE_Pages table. Only a single row is ever returned though, and in the comments you said:


  most of the time it['s result set] is only a selection of a few thousand


Something is going very wrong with cardinality estimation. From the optimizer's point of view, it's giving you the best plan that it can. It thinks that all plan options are expensive because billions of rows need to be returned. When returning all rows, in general, a clustered index scan will be costed much cheaper than using an NCI + key lookups to return every single row. The bad cardinality estimate results in a query plan that does the scan which takes days. The key to getting better performance without index hints is to fix the cardinality estimate.

You said that this problem seemed to start happening after you upgraded from SQL Server 2016 to SQL Server 2017. Did you gather statistics after the upgrade? The two most likely causes are that there's something wrong with the statistics for dbo.SE_Pages or Microsoft made a change in SQL Server 2017 that's harmful to your workload.

You can test if statistics are the cause by updating them for that table. You can test if SQL Server 2017 is the problem by changing the compatibility level of the relevant database to 130, although this could have other effects on your application. If you need another temporary workaround that doesn't involve an index try FAST 1. That hint encourages the optimizer to prefer plans which return the first row as quickly as possible. In your case, it's a way to disfavor plans that do a scan. If you need additional help you'll need to add more information to your question. The histogram for UriKey and the table structure (along with an explanation as to why you're partitioning by UriKey) are likely to be helpful.