Just something to point out: it’s been confirmed by Jeto that the twinblade is calculated at the end of a turn, not beginning, so essentially any “start of turn” effects that generate gems will not effect. Here is her explanation:
Now her verbiage is not explicit as to “end of turn” but her description is essentially that all actions are resolved in a matching phase and once everything is tabulated, then the crit chance of the twinblade is calculated. For a start of match order of precedent, the twinblade would calculate based on the starting board and then start of turn effects like Lucky Clover Ring, etc. would apply.
I have not noticed this but I will check it out myself as well. This is the one description that seems to run counter to what was intended. Are you using the Mercenary Gems before matching phase or while the matching phase counter is running down? If its the latter then the % chance is being recalculated at the end of the Speed timer and the newly resolved board state which might be the reason you are seeing the chance fluctuate.