It is possible that the plates are gaining speed due to sulfur in the formula or due to reduction sensitization from any reducing agent in the formula.

This can happen if the formula contains no stabilizer. It depends and not knowing the formula makes diagnosis difficult. Using those japanned aluminum plates does not help. Generally, aluminum is not good with photo products such as emulsions. Some lacquers contain harmful chemicals as well.

Try it on a plain glass plate and see what happens.