Was the investment in pound sterling? The pound has been going down steadily over the last year. And now we have a steep drop due to Brexit.
Am not speaking from a position of particular knowledge on this, but the way I read original the email was simply that income was held in some kind of depsit account in the currency in which it was received - so GBP income remained in GBP until there was a need to spend GBP. WMF does after all have significant expenditures in other currencies than dollars (e.g. grants and salaries of overseas staff).
Regards,
Chris