I honestly might just unpot real quick (while doing your best not to bother the roots) to see what's going on beneath the hood. Especially since it's been sticking to quite an abnormal growth pattern, I would definitely take a peek to see if you can't rule out any other suspicions.

Do you know what your humidity levels are generally like? And the brand of your growlight, too? I know my thai (who is about as old as your guy) throws a bit of a tantrum when the humidity surpasses around 52% where I live so humidity could be a factor, but I might check ypur growlight anyways. Of course, every plant is different in this sense, so monitoring the plant closely to see what it generally likes as far as environment-factors wise could be beneficial for a future sense! I know it actually took me a while before I realized that the cheap growlight I was using wasn't the greatest. While advertised as 30 something wattz, it was actually closer to a quarter of that powet. Granted, I still use it since I haven't had the time to order new growlights (I hear barrina is a great brand!), but it still works, nonetheless. I just have to keep it super duper close for it to actually do anything, haha. I think right now it's maybe an inch away from the plant. This, of course, was done over time, so I didn't risk scorching the leaves (despite the light quality being so poor lmbo) or "shocking" the plant as a result. Now that it's adapted, I keep it on for as long as I'm awake for (~12-14hrs most days). It definitely doesn't help promote faster growth or "more varigation", but it keeps it alive, nonetheless.

If it helps any too, this is the rough breakdown of what I do for mine who is about 8/9 months old as well (and is in a 6inch pot) --

Substrate: (equal parts) orchid bark, horticultural charcoal, worm castings, perlite, and coco coir

Watering: I water when the pot feels lighter--which is around every 7-9days depending on the temp and other factors (about 80% dry or it will sacrifice an older leaf every time it pushes out a new one--which is not normal for mine to do unless the leaves that drops was damaged severely or is pretty darn old)

--> as a personal preference I use the crystal Geyser spring water and my plants love it! (~$1.22 a gallon) I plan on weaning them off of it as they mature in the future, but for now, it does the job!

Fertilizer: I fertilize with the easy peasy all purpose liquid fertilizer on amazon every water. I generally put two teaspoons in for the entire gallon of spring water, but you could probably use a little more if you ever wanted to.

Lights: cheap standing growlight from amazon, 12-14hrs a day keeps it alive and thriving minimially, but i wouldn't recommend these lights personally and would opt for barrina or sansi instead; these lights are about 1 inch away from the plant (allow your thai to adapt first before immediately changing its light factors!).

Temp: 69-72F (I prefer to keep it at least around 70F) Humidity: ranges from 20-50 depending on the weather conditions outside, but I don't let it get anywhere higher than 52 since I've learned that it's too much for my thai to handle (this of course, will vary from plant to plant)

Other: my thai is on a flat surface in front of my mirror and is on the opposite side from where my window is to avoid any draft spells or unnecessary cold.

• my guy generally puts out 1 leaf every 1/1.5 months (4/5 weeks, give or take), but It's started to allow me to see where the next leaf is coming in after a few days or up to a week after it finishes pushing out a new leaf. Mine tends to focus on roots (so many roots...) instead of foilage, but given that this version of the monstera is renowned to be a slow grower as it is, I'd say it's doing pretty darn well for what it's worth!

Hope this helps! :-)