It seems a wrong Dcodes be assigned. You may try to creating Dcodes for this layer to check it.
In your CAM setup for the soldermask layers check the "Options" and check for an 'oversize' value. I f you have already defined your soldermask pad in the padstack, then you generally do not want the CAM routine to add an oversize value, Or the oversize value may simply be excessive for this application.
Thanks DenisL, sorted!!
Keep in mind, that you have to ensure min 0.1 mm solder web. Also, you have to regenerate D-codes in CAM output.