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Source: https://apps.dtic.mil/sti/tr/pdf/AD0155208.pdf
Note that the 1953 US MIL-A-12560 standard for RHA called for a V-Notch Charpy impact toughness at -40°F for a 6in plate of at least 28ft*lb. Much lower values of this specimen have probably made it much more susceptible to an attack from a HESH round (among other things).
Unlike other flaws of RHA like laminations or non-metallic inclusions, armour with low impact toughness will show a reduced ballistic resistance only against high caliber shells like the soviet 122mm and 152mm. Against small(-ish) caliber with high penetration, like 17pdr APDS or even full caliber 17pdr and US 90mm AP shells, it would’ve likely performed as well as one with adequate level of impact toughness.