In the step0.pt file in stemmers/rslp.zip the following line:

"ães"	1	"ão"	"mãe"

has mãe as an exception for the ães rule. However, the implementation requires the exceptions to be in their plural form (which, in this case, would be mães). A simple fix for this would be to change the line into

"ães"	1	"ão"	"mães"