Part of your confusion is that you are confusing "spall" and "splash".
is when pieces of the interior armour surface are fractured off and projected around the inside of the vehicle (not the support structure, the plate itelf). It can happen to any vehicle and is generally countered nowadays by Kevlar spall liners and/or spaced armour.
occurs when lead cored small arms bullets strike an AFV. The impact generates a large amount of heat, melting the lead core. The copper jacket is split by the impact and allows the molten lead to spray along the path of the impact. Old rivetted vehicles and vehicles with any straight through path for the molten lead are prone to having the molten lead spray into the seams between plates and through the gaps. It sprays over the crew inside and causes a lot of minor injuries. The main not-so-minor injury was blindness as the crew were frequently looking through view ports that were the main targets of enemy gunners to cause exactly those injuries. This effect was lessened by the use of eye and face guarding goggles in WWI, but was really fixed when vehicles were welded and had splash guards and lips added around all openings so that the spray couldn't work its way inside. All accounts point to the fact that this splash was quite incidious in finding every possible crevice and seam.