Not sure what is really going on there, but one problem you might be experiencing is the effect of tin dissolving the copper of your soldering tips.
Tin, when in a liquid state, readily dissolves other metals such as copper, silver and gold. Tips are usually coated with a layer of iron for durability, but if that iron layer gets breached (eg. cracked or ground / scratched through) it is only a short time before the copper begins to dissolve into liquid-solder solution. Damaging the iron layer is a major reason a lot of people caution against grinding or filing your solder tips to reshape them. Since SAC305 solder is 95% tin (and the working temperatures are higher), it will dissolve copper far faster than standard 63/37 SnPb. I've seen it happen on component leads many times.
Google "dissolution of copper in lead free solder alloys"