The appellant shall not, except by leave of the Appellate Board, urge or be heard in support of any ground not set forth in the memorandum of appeal, but the Appellate Board, in deciding the appeal, shall not be confined to the grounds set forth in the memorandum of appeal or taken by leave of the Appellate Board under this rule:

Provided that the Appellate Board shall not rest its decision on any other ground unless the appellant who may be affected thereby has had a sufficient opportunity of being heard on that ground.