When you cast the same spell repeatedly as a result of casting that spell “additional times”, the additional casts also count as “manual” casts. For example, casting a spell 3 times as a Spellweaver using Eternal Echo granted 3 infusions. After adding the “% Chance to Cast Twice” spell gem property and casting the spell 4 times, this increased to 4 infusions/manual casts.
However, if the initial cast triggers the Chrono Shift and/or Temporal Anomaly perks and causes your other gems to cast, the additional casts do not count as manual: The very first cast counts as manual, then the above & below gems cast three times each as non-manual, then the first spell casts its remaining two times as non-manual, for a grand total of just one manual cast.
I’m not sure which of these two behaviors is the bug, honestly, as until now I assumed that “additional” casts did not count as “manual” casts. Either way, it leaves Spellweaver in a weird place where Chrono Shift and Temporal Anomaly hinder the other perks from triggering as often.