@dfrausto, Thank you for the update. It does appear that the issue with Sinhala and the issue with Amharic are one and the same, but I’ve only just realised this. This problem in Windows 10 1803 is something that we have been chasing with Microsoft. You can see more technical details on the problem here: https://github.com/keymanapp/keyman/issues/1285
The mitigation at this time is to assign the Amharic keyboard to the Ge’ez language, by following the steps in this post: Many Amharic keyboards not working
You may also want to upvote the Microsoft feedback issue to flag that you are experiencing the problem: https://blog.keyman.com/2018/10/how-to-send-feedback-to-microsoft-for-issues-that-impact-keyman/
(Note I linked to the instructions for generating a diagnostic report in my previous post, but given this additional information, you won’t need to do that).