Mysterious current appearing on Ltspice using LT6018 opamp
when the datasheet says it can output max 50mA. Even if it has to saturate, it should saturate at 50mA.

No, the 50 mA is the maximum rating that  means, you can expect reliable performance when you do not exceed that 50 mA. Look further in the datasheet and see how \$I_{SC}\$ the short circuit current is 90 mA or 100 mA which is larger than 50 mA!
So the opamp can output more that 50 mA. But that exceeds the maximum rating and that means it will have an impact on reliability. Meaning, the opamp might not last as long as it should (if you stay below 50 mA).
Also you cannot rely on every aspect of an opamp to be modelled correctly. It is also up tou the designer to make sure that the output current never exceeds 50 mA.
Regarding your circuit: You have a large capacitive load directly to ground on the opamp's output, \$C_{act}\$ and \$C_{sens}\$ in series will be just below 700 nF. Opamps generally don't like this, there has to be some series resistance. Also stability might not be guaranteed. I think you already see that in your simulation result as the behavior is weird.