Empty meta description for some posts when %%excerpt%% used

Update: I have now determined that the meta description tag only ends up being empty for certain posts – not sure why, no obvious differences in content – I will try to debug this when I get a chance.

Original post:
I am trying to set a default meta description template for posts using the %%excerpt%%

However, once I include %%excerpt%% in the template, I get an empty meta description tag in the head for certain posts.

I can include other variables fine e.g. %%sitename%% or %%sitedesc%%, but once I include %%excerpt%% along with those (or just on its own) I get an empty meta description tag on certain posts.