Figma is crushing Adobe XD, not Adobe. But we will never know the exact reason for the increase in R&D spend - but without the increase in revenue they would not have been able to afford this investment in R&D (R&D is approx 60% of their revenue).

Again it's impossible to define cause and effect, but OP's claim that they are investing less in product after introducing subscriptions is spurious - R&D has dramatically increased almost exactly in line with revenue.