These attacks were so successful that Russia is having fuel shortages right now, and decreased its exports of oil products by about 10%. It is estimated that about 30% of Russia's refining capacity is disabled by now. (E.g. https://www.bbc.com/news/articles/czx020k4056o)
So no, these are not isolated incidents, this is a big hole in air defense, and Russia used to produce some competent air defense systems. They are just not geared towards the drones, much like Patriot air defense systems are not either.
I agree the attacks were successful, I just don't agree they were successful because of lack of detection.
You generally can't continually launch a even $1mil missiles at $100-$10k drones and expect to win. The problem is that most air defenses were designed in an age where the enemy aircraft were quite expensive so it wasn't so critical to optimize ordinance cost. That IMO is the primary challenge for all militaries now.
You don't need a missile. Any fighter jet is equipped with a cannon; a bunch of 30mm fragmentation-action ammo would dispatch the drone. You don't need a jet; any decent piston-engine trainer plane can fly 4x as fast as the drone, just mount a machine gun on it. Hell, take a civilian helicopter, shoot through the open door. You have ten hours to dispatch it, given that you have detected it.
But, since this is not being done routinely along the way, I conclude that detection fails. All reports about drones shot down mention that the drones were shot down nearby, likely by local air-defense teams near the target. These could as well detect the drone visually, or by the sound.
The Russian air defense radar system has been badly degraded by recent Ukrainian strikes. They have been conducting a deliberate SEAD/DEAD campaign to clear the path for strikes on strategic targets. Of course the Russian air defenses everywhere far from Moscow were probably never very effective in the first place due to the usual mix of poverty, corruption, vodka, and incompetence.
So no, these are not isolated incidents, this is a big hole in air defense, and Russia used to produce some competent air defense systems. They are just not geared towards the drones, much like Patriot air defense systems are not either.