i got this baby philodendron (im not sure exactly what kind - not sure if anyone knows?) in a sustainability box from plnts in july last year and its previously been super happy but recently one of its leaves turned yellowish beige and i feel like it just looks a bit floppy! pic 5 shows it looking imo a bit perkier a few months ago, and pic 6 is from when i first got it last july! 

its in a south-west facing window, and i water it when the soil dries out completely as shown on a moisture meter (paranoid about overwatering) 
i fertilised it a couple of weeks ago but before that never had
i potted it with the plug in soil when i got it and repotted it into a chunkier mix a few months ago

one of its leaves accidentally got knocked off so i dont know if it could be that? 
i just feel like the roots have never PROPERLY spread throughout the pot - i feel almost like i could quite easily still pull the plug out if i applied any force at all, i dont know if its in too big of a pot?
could it be sunburn from being too close to the window? it always previously looked happy there but its gotten sunnier recently
could it be that the old leaf is dying off because its pushing up that new leaf?
could it be that im not watering it enough? or too much?

if anyone has any advice on getting baby plants properly established or on how best to care for this plant i would be so grateful!!! i rly want to keep her happy


advice on baby philodendron!