As path based metatags are entities the field based translation can be used. However, since this entity-type has it's very own language handling using field based translation leads to trouble.
A scenario in which that happens is when a translatable meta field is used in nodes with field based translation enabled.
As soon as you use this translatable field for path based metatags the values of the field are stored with a specific language that may not matches the language selected for path based metatag entity itself.
As soon as the path based metatag entity is load in the frontend this inconsistency between entity and field language leads to missing field values.
I think the fastest/easiest fix would be to enforce the field language for path based metatags to
The attached does exactly this and provides an upgrade path too.
Another solution could be to introduce proper translation support, but I don't think that there would be any benefit.
Review of the approach and the code necessary
User interface changes