Christmas Light Electrical Question?

I had a question. I have sets of 50 light mini-lights broken into 2 separate 25 light sections. I could not find a good source for economical replacement lights for 25 long sections, which I presume would be 5v lights. The package says that the full string of lights of 50 bulbs is 40.8w.

I found some light strings with 20 lights per string and figured if I used them as replacement bulbs it would do no harm since they should be 6v lightsand figured the set would just burn a bit dimmer as a result of these being in the line.

It seemed however, that at some point where I got multiple 6v replacement lights in the strand, that the whole string would "flash" taking out most of the lights. Question is, did I error in my logic, or perhaps did the bulbs just randomingly reach the point where they were old enough to start the chain reaction failure?

Second question, If I wanted to replace all 25 with new 6v shouldn't they then be a dim string that would last longer due to the lower current flow?

Thanks!