Why does this happen?

Glib as this may seem to be, it is *not* remotely meant so:

"Granted, octaves are a strength of my playing, but they are physically more difficult than close-position block chords, which I have been able to handle since my first few weeks of playing the piano."

Familiarity breeds contempt.... (It's a common hazard of all motor skills acquired by repetition: once conquered, they become 'furniture' and that's where the temptation of structural inattention starts to wreak havoc, given half a chance...)

There's nothing more sinister than that going on there, I'm sure.

Loively to hear, see & read you're back! :-)

Warmly, as always,

Most hands will fit an octave very nicely and playing octaves usually only use finger 1-5 or 1-4. Playing block chords means you need to fill in notes in between the octave and the fingering for the inside notes have several variations. Easy to miss them and will require more concentrated practice to make the jumps and land correctly on the full chord.

I am not sure if this was your question. Sorry if I missed your point.

I just relax my body and give all the strength needed from my shoulder to the fingertips. Relaxing is the main thing. Well, for blocked chords, you should play them slowly first. Then, once you are confident everything is fluent, you can start speeding up, and i always listen to the notes I'm playing and pay attention. It takes patience and time to practice piano.

It's probably because your hands are not independent enough yet. Reading the music is dealt with the left hemisphere of the brain, which also controls the right hand (that's how it's wired up). While the left brain is occupied with reading the music, it neglects right hand function.

I made that up by the way, but it's probably something like that...

Because you pay more attention when playing difficult passages and get lazy with the "easy" parts.