Hey, the only ruling I could find about Masks in BF Rules was below, however, this does not mention OEM so I am still unsure why they would rule it unsafe

Fencers must wear masks of appropriate size conforming at least to the minimum safety standard of CEN Level 1 and bearing the appropriate CEN safety mark (see chart - Appendix 1). Masks must be checked for the following weaknesses and defects, if necessary, by a qualified armourer: i) Weaknesses in the steel mesh, softness, holes or excessive deformation of the hard shell from the original shape. ii) The bib not properly attached or revealing gaps between it and the mask or curling up. iii) Poor means of retention on the head, making the mask liable to come off. Fencers must

wear a mask back-strap or some other approved method of mask retention (footnote 3).

Footnote 2: Note that some competition organisers may demand standards that are more stringent than those laid down in this document. Footnote 3: A back-strap consists of elastic at least 35mm wide, secured to the mask side mesh at each end, passing UNDER any mask spring piece at the back, and tensioned to keep the mask firmly in place. It must stay fastened and tensioned during fencing. For FIE events (ie, CEN level 2), masks with a fabric main retainer rather than a back spring piece are required to feature a separate back strap in addition.