When a word has two or more constituents we often hyphenate them. A word so hyphenated is considered single for all general purposes; e.g. well-wisher, seven-year-old etc. These words appear as well-wishers and seven-year-olds in their plural forms.
On the same analogy we tend to make the plural of any hyphenated word by adding an 's' to the ending component. For instance, the plural of 'brother-in-law' is made 'brother-in-laws' rather than 'brothers-in-law', though the dominant word within is 'brother'.
Such pluralizations are not standard and correct, particularly in BrE, but a trend to use it that way is sometimes found. Is it acceptable?