It found such a correlation only sometimes, and at any rate such a correlation is either irrelevant or harmful to the actual claim of interest (that mask mandat

It found such a correlation only sometimes, and at any rate such a correlation is either irrelevant or harmful to the actual claim of interest (that mask mandates have a negative effect on COVID). The correlation between masking and reduction of COVID is zero. It certainly doesn't imply the existence of a confounder that takes real effectiveness and reduces it somehow past zero. Once you try to explain the (inconsistent) positive correlation between masks and COVID you're back in the territory where "correlation != causation" is a valid counter argument. As you note, such a correlation could be driven entirely by the false perception of effectiveness, without any actual effectiveness. But it can also be for other reasons e.g. places that were tougher about enforcing mask mandates had generically less competent governments that were more likely to do harmful things like flushing hospital patients into care homes. More likely: such correlations are spurious and there's no real causality there at all. The fact it doesn't show up consistently would certainly imply that. Also possible that the IHME mask usage data is bogus in some countries, or potentially lots of other things. We have no evidence that can support any explanation over any other.