When reviewing first instance proceedings the boards' review is, in principle, not limited to fundamental violations of the right to be heard.
However, remitting a case to the department of first-instance under Art. 11 RPBA 2020 also requires a "fundamental" deficiency in first instance proceedings, and under R. 103(1)(a) EPC the appeal fee is only reimbursed in case of a "substantial" procedural violation. In T 689/05 the board linked these concepts, stating that a "fundamental" deficiency within the meaning of Art. 11 RPBA is not caused by all procedural violations but rather only by a "substantial" procedural violation. In J 7/83 a substantial procedural violation was defined as an objective deficiency affecting the entire proceedings, in T 682/91 as a deficiency adversely affecting the rights of the parties (see chapter V.A.11.6.2 "Violation must be substantial and affect the entire proceedings").
In T 990/91 the lack of opportunity to reply to a supererogatory and incidental argument put forward by the examining division could not be considered to be a violation of the right to be heard, let alone a substantial one.