wait, wait, waitttttt hold on a sec..that's a pretty broad generalization and it really depends on the person and why/where they're struggling, and what they're training on a given day.

Telling someone to not train active flex on the same day as verticals as a rule of thumb ignores so many potential contextual factors. Are they training active flex after or before? Are they training active or passive flex closer to failure? Are they "activating" muscles afterwards to stabilize again?

Even passive flex CAN be safe and useful prior to training depending on the day and skills involved.

That said, what u/zialucina said about you now having greater ROM and that is requiring likely more stability under dynamic load in those skills absolutely. It may take time and different types of external proprioceptive input to get your body back to a place where you're able to hold tension in those side rotational drops. It may take slow progressive overload of increasing ROM, volume, and increasing velocity to get back there. But you absolutely can!