Why are these guys yellowing at the base, stem and all?

I've had these monstera thai both for about 7 months now from sprouts, they've grown triple the size and put out fenestrated leaves, the second one even had huge new growth, both did well for a while but I've noticed that both plants have lost their two bottom leaves, with both going completely yellow.

I do fertilize every 2-3rd week with diluted foliage pro, I water once the mix is usually almost completely dry (I use a chunky mollys aroid mix) which usually comes out to every 7-9 days.. both have air roots that are going bananas.

Any advice as to what I should trouble shoot here?