Version 1.8 of the OpenType specification was released in September 2016 and has significantly revised the following features: Initial Forms {init}, Isolated Forms {isol}, Medial Forms {medi}, and Terminal Forms {fina}. They should no longer be used with Latin script. Most OpenType layout engines haven’t been updated yet, but sooner or later they might drop support for Latin based init, medi, and fina.
To be sure your Latin fonts continue to work, we recommend to use a different approach to keep in line with the specs.
This topic explains how to use chained context substitution to mimic init, medi, and fina for Latin based characters: