It sounds like @lansalot might have been pushing an old config file to Yealink phones with newer firmware, and that old style config MIGHT have been slightly incompatible - and caused the issue. which @lansalot was able to resolve, not by completely modernizing his config files, but instead by adding a few lines to expressly bring OPUS back into the config.
That should not matter. If the config is old and does not have the setting specified at all, it should go to the default value.
Sure, of course you'd like to think so - but bugs do happen... and it appears that it is in this case.