No worries at all! With the amount of mechanics in Serenitrove, sometimes I forget things too :)
I tested Iron Sigil combined with the Siphon Pickaxe on Farfields metal ores using Fern, and they do seem to be stacking correctly. On iron ore for example, it shows a stamina restore of +10 (which equals +11 when factoring in the -1 stamina cost for digging). Tested it on cobalt ores as well.
It might have been related to Fern's overhealing mechanic (restores 50% of the stamina gain instead of 100%). Try lowering Fern's stamina slightly before digging metal ores, and you should see the correct restoration amount.
And yes, I agree the Siphon Pickaxe feels a bit overpowered right now. I plan to increase its crafting/repair cost while also slightly reducing its stamina restore, maybe +5 or even +4.
You're very welcome! I was fully prepared for extensive bug fixing and balancing sessions with the Farfields update. There are just so many new mechanics, biomes, and pieces of equipment being introduced.