Keeps switching away from the keyboard source input I select

Hi, I have recently found an input source that allows me to type characters in Yoruba which aren't available natively. I have added this input source to the "Keyboard Layout" folder and have added it to the list of input sources in Preferences > Keyboard > Input Sources, also on this page the box saying "Automatically switch to a document's input source" is unchecked as seen in the attached photo below.

The problem is it looks like I can only use it in specific apps I am able to use it fine in chrome however when I try and type the character in messages it automatically switches back to the default English keyboard. I copied across the characters and messages recognises them so it's not that they're incompatible. I looked to try and remove the English default from the Source Input but it doesn't seem to allow that either.

An example of some of the characters are ọ ẹ ṣ̀ for instance, and like I said work fine through chrome but nowhere else it seems. I can see in the top bar the keyboard input switch back to English. Any help would be greatly appreciated!