Manganese does not split hydrogen from water molecules. I think you might have it mixed up with magnesium, which does but not at a high enough rate to release an explosive amount even if it's liquid water and solid magnesium.

Sodium and lithium do release enough hydrogen to be explosive and enough heat to ignite it when they are solid in liquid water, but I suspect the gases wouldn't do that. The density is way lower, and if it does prefer stripping hydrogen from water molecules to just pairing up with the the O2 ones (some reading suggests that this is the case for solid sodium exposed to air), I bet the recently freed hydrogen will take that heat and just join up with the free O2 itself before it has a chance to build up. And the source would also be more spread out, so there wouldn't be any reason for a high concentration to form even if the hydrogen doesn't react immediately with O2 in the vicinity.

Not that welding isn't an explosive risk, but it's usually due to the tanks used directly by the welding and/or explosive/accelerant materials in the vicinity of the weld.

If someone offers you $80k to weld their fertilizer tank because it's too full and they want to add an extension, politely decline and consider avoiding the general area until after it explodes.